Add a Popover widget, that shows a widget in an overlay (#100)

This serves a similar purpose to `GtkPopover`. It takes two widgets, one
to show normally and one to show in an overlay.

This should basically work, though more options could be added here.

src/widget/mod.rs

@@ -23,6 +23,9 @@ pub use nav_bar::nav_bar;
 pub mod nav_bar_toggle;
 pub use nav_bar_toggle::{nav_bar_toggle, NavBarToggle};
 
+pub mod popover;
+pub use popover::{popover, Popover};
+
 pub mod rectangle_tracker;
 
 pub mod search;