Initial wl_touch support

`builtin_output` is similar to `get_builtin_output_name` in Sway.

For full support, we need support for touch targets/grabs, touch support
for SSDs, and a way to configure what output is mapped to what input.

But this is usable for apps that support touch on a laptop with a
touchscreen.

src/shell/mod.rs

@@ -1326,6 +1326,13 @@ impl Shell {
             .map(|(o, _)| o)
     }
 
+    pub fn builtin_output(&self) -> Option<&Output> {
+        self.outputs().find(|output| {
+            let name = output.name();
+            name.starts_with("eDP-") || name.starts_with("LVDS-") || name.starts_with("DSI-")
+        })
+    }
+
     pub fn global_space(&self) -> Rectangle<i32, Global> {
         self.outputs()
             .fold(