feat(quickshell): Alt+Tab overview of every monitor and workspace

Each monitor is a row of workspace cards at its own aspect ratio, with
live captures of the windows where they sit, including windows on
hidden workspaces. Alt+Tab opens it with the previous window picked,
further Tabs walk the focus history, and releasing Alt switches.
Super+W opens it without the hold. Hover, click, Enter and the
workspace digits work too.

Group cycling moves from Alt+Tab to Super+Tab and Super+Shift+Tab.

quickshell/shell.qml

@@ -7,6 +7,7 @@ import "root:/bar"
 import "root:/launcher"
 import "root:/dashboard"
 import "root:/ask"
+import "root:/overview"
 import "root:/overlays"
 import "root:/lock"
 
@@ -18,6 +19,7 @@ import "root:/lock"
 //   popouts/   hover panels hanging off bar modules
 //   launcher/  the application launcher
 //   dashboard/ the drawer that drops from the bar
+//   overview/  every monitor, workspace and window at once (Alt+Tab)
 //   ask/       the question panel, dropped from the top edge
 //   overlays/  power menu, notification toasts, OSD
 //   lock/      session lock
@@ -84,6 +86,12 @@ ShellRoot {
         AskPanel {}
     }
 
+    Variants {
+        model: Quickshell.screens
+
+        Overview {}
+    }
+
     Variants {
         model: Quickshell.screens