feat(borders): add border manager module

This commit removes all border-related code from process_command,
process_event etc. and centralizes it in the new border_manager module.

Instead of trying to figure out where in process_event and
process_command we should make border-related changes, a notification
gets sent to a channel that border_manager listens to whenever an event
or command has finished processing.

The border_manager listener, upon receiving a notification, acquires a
lock on the WindowManager instance and updates borders for the focused
workspace on every monitor; this allows us to centralize all edge case
handling within the border_manager listener's loop.

Borders on workspaces that lose focus are now destroyed and recreated
when those workspaces regain focus, instead of trying to share
individual border instances across workspaces.

A number of common edge cases that have been addressed in this commit
are:

* Paused window manager
* Floating workspaces
* Maximized windows
* Fullscreen videos
* Monocle containers
* Ghost borders on workspace switching
* Incorrect focused window border colours

Global state related to borders has also been moved into the
border_manager module, which also tracks the state of border objects
(BORDER_STATE), their rects (RECT_STATE) and their focus kinds
(FOCUS_STATE).

This allows us to now track multiple borders per-container, enabling
unfocused border windows for the first time.

Additionally, the Z-Order for border windows is now also configurable.

ActiveWindowBorderColours has been expanded to include Unfocused, but in
order to not introduce a breaking configuration change for end users,
all members of this struct have been made Option<Colour>.

komorebi-core/src/lib.rs

@@ -218,6 +218,7 @@ pub enum WindowKind {
     Single,
     Stack,
     Monocle,
+    Unfocused,
 }
 
 #[derive(