Migrate Budgie toolbar code to BudgieControlBar

This renders the toolbar unusable for now, as there is no signal
connection, but it does offer far better grouping of UI components
within Budgie.
Initial signal from BudgieControlBar
Ensure we unref our GtkIconTheme
Link each button to BUDGIE_ACTION enum
Link methods up to BudgieControlBar
Remove old callbacks in BudgieWindow
Reenable full-screen control
Synchronize aspect_cb with BudgieControlBar API
Add method for hiding or showing video controls

We do not wish to show our video playback controls during normal
audio playback, so we'll set them to show if we're playing video
only.
Add functionality to enable/disable toolbar actions

Currently we only allow reload to be enabled or disabled but the
function allows for future extensions
Add function to set an action (button) as active or inactive

This is used to update the full_screen button state currently, but could
quite easily be expanded in the future.
Fix BudgieControlBar dispose issue
Add new non-functioning playback controls to BudgieControlBar
Only show toolbar playback controls in full screen mode
Link actions up to BudgieControlBar
Come out of full screen when videos end
Move BudgieWindow prototypes into budgie-window.h
Remove no-longer-true comment
Add BudgieAction settings launcher to BudgieControlBar
Add BudgieSettingsView skeleton

We'll use a nice settings view within our GtkStack instead of additional
windows.