Add support for workspace pinning and dragging

Workspaces can be pinned, and dragged to reorder or move to a different
output.

These features are enabled only if cosmic-workspace-v2 advertises the
necessary protocol version and capabilities.

The layout of the labels and pin buttons could be tweaked a bit still.
Some hacks and workarounds are needed to get drag and drop working as
desired. Something iced and libcosmic could potentially improve in the
future. But this now seems fairly robust.

src/widgets/match_size.rs

@@ -0,0 +1,145 @@
+//! Show one surface, sized to match the size of another (invisible) widget
+
+use cosmic::iced::{
+    advanced::{
+        layout, mouse, renderer,
+        widget::{Operation, Tree},
+        Clipboard, Layout, Shell, Widget,
+    },
+    event::{self, Event},
+    Length, Rectangle, Size,
+};
+use std::marker::PhantomData;
+
+pub fn match_size<
+    'a,
+    Msg,
+    T1: Into<cosmic::Element<'a, Msg>>,
+    T2: Into<cosmic::Element<'a, Msg>>,
+>(
+    matched: T1,
+    shown: T2,
+) -> MatchSize<'a, Msg> {
+    MatchSize {
+        matched: matched.into(),
+        shown: shown.into(),
+        _msg: PhantomData,
+    }
+}
+
+pub struct MatchSize<'a, Msg> {
+    matched: cosmic::Element<'a, Msg>,
+    shown: cosmic::Element<'a, Msg>,
+    _msg: PhantomData<Msg>,
+}
+
+impl<Msg> Widget<Msg, cosmic::Theme, cosmic::Renderer> for MatchSize<'_, Msg> {
+    delegate::delegate! {
+        to self.matched.as_widget() {
+            fn size(&self) -> Size<Length>;
+            fn size_hint(&self) -> Size<Length>;
+        }
+    }
+
+    fn operate(
+        &self,
+        tree: &mut Tree,
+        layout: Layout<'_>,
+        renderer: &cosmic::Renderer,
+        operation: &mut dyn Operation<()>,
+    ) {
+        self.matched
+            .as_widget()
+            .operate(&mut tree.children[0], layout, renderer, operation);
+        self.shown
+            .as_widget()
+            .operate(&mut tree.children[1], layout, renderer, operation);
+    }
+
+    fn on_event(
+        &mut self,
+        tree: &mut Tree,
+        event: Event,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        renderer: &cosmic::Renderer,
+        clipboard: &mut dyn Clipboard,
+        shell: &mut Shell<'_, Msg>,
+        viewport: &Rectangle,
+    ) -> event::Status {
+        self.shown.as_widget_mut().on_event(
+            &mut tree.children[1],
+            event,
+            layout,
+            cursor,
+            renderer,
+            clipboard,
+            shell,
+            viewport,
+        )
+    }
+
+    fn mouse_interaction(
+        &self,
+        tree: &Tree,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        viewport: &Rectangle,
+        renderer: &cosmic::Renderer,
+    ) -> mouse::Interaction {
+        self.shown.as_widget().mouse_interaction(
+            &tree.children[1],
+            layout,
+            cursor,
+            viewport,
+            renderer,
+        )
+    }
+
+    fn layout(
+        &self,
+        tree: &mut Tree,
+        renderer: &cosmic::Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        // TODO?
+        self.matched
+            .as_widget()
+            .layout(&mut tree.children[0], renderer, limits)
+    }
+
+    fn draw(
+        &self,
+        tree: &Tree,
+        renderer: &mut cosmic::Renderer,
+        theme: &cosmic::Theme,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        viewport: &Rectangle,
+    ) {
+        self.shown.as_widget().draw(
+            &tree.children[1],
+            renderer,
+            theme,
+            style,
+            layout,
+            cursor,
+            viewport,
+        );
+    }
+
+    fn children(&self) -> Vec<Tree> {
+        vec![Tree::new(&self.matched), Tree::new(&self.shown)]
+    }
+
+    fn diff(&mut self, tree: &mut Tree) {
+        tree.diff_children(&mut [&mut self.matched, &mut self.shown]);
+    }
+}
+
+impl<'a, Msg: 'a> From<MatchSize<'a, Msg>> for cosmic::Element<'a, Msg> {
+    fn from(widget: MatchSize<'a, Msg>) -> Self {
+        cosmic::Element::new(widget)
+    }
+}

src/widgets/mod.rs

@@ -19,6 +19,8 @@ mod toplevels;
 pub use toplevels::toplevels;
 mod visibility_wrapper;
 pub use visibility_wrapper::visibility_wrapper;
+mod match_size;
+pub use match_size::match_size;
 
 // Widget for debugging
 #[allow(dead_code)]