panel: Hook menu button up to new MenuWindow
panel: Add menu theming

We use an RGBA visual on the panel to allow rounded windows and
any transparency that a user may wish to apply via CSS theming
menu-window: Populate view with categories and entries
menu-window: Size and placement fixes
menu-window: Enable filtering by category
menu-window: Display category names in All view
menu-window: Add margin to elements
menu-window: Use display name for entries
menu-window: Allow searching to filter
Hide menu window when lost focus or escape is pressed.
menu-window: Now we launch applications
menu-window: Grab focus when we display
panel: Cleanup. Also add tooltips to menu items