polybar: Add pop-up calendar to date module

Added calendar (using gsimplecal) which appears on left-clicking the
date module. Previous long date format switching has been moved to
right-click.

Under the hood, the official date module has been exchanged with a
custom script which simply replicates the functionality but allows
arbitrary commands to be executed on interactions with the module (since
it is now a script module).

bootstrap/packages.txt

@@ -37,6 +37,7 @@ gnome-keyring
 go
 gopls
 grub
+gsimplecal
 hugo
 i3-gaps
 i3blocks