panel: Add BudgiePopover skeleton
popover: Add initial drawing code

This essentially draws a little tail/arrow at the bottom center of the popup,
and makes too many assumptions to be generally useful.
panel: Add BudgiePopover to build
panel: Make MenuWindow use BudgiePopover

BudgiePopover is a simple popup type window, which has a "tail", or arrow,
pointing at the relevant widget. This is far more friendly and visually
appealing than a semi-transparent black window.
popover: Add _present() method

The present method will be used to show the popover on screen, relative
to the parent widget. The assumption is still made that the popover should
appear *above* the widget, not anywhere else.
panel/applets: Use budgie_popover_present in MenuWindow
panel: Ensure BudgiePopover is located correctly

BudgiePopover will now be located correctly, and draw the tail in the
right position, for the menu applet.
Panel: Add click functionality on the application buttons