This commit decouples the surface height from the usable height, making it
possible to draw a separator between the workspace/mode and window borders
on top (for bottom bars) or bottom (for top bars). The workspace and mode
borders also get separated.
This behavior is consistent with i3bar.
This removes any pending messages once the item is destroyed.
Furthermore, this installs SNI event calbacks asynchronously
in order to prevent sd-bus from bypassing pending messages.
In case a tray icon cannot be found or does not have a desirable size,
swaybar retries the search again and again, which increases load on disk
and CPU. This commit solves it by storing target_size for each icon, so
that swaybar does not search for an icon of some size if it already tried to.
Fixes#3789.
