applets/mediacontroller: Fix contextual actions behavior

Set the context actions once and use bindings to update their status, instead of resetting them just when showing the context menu.

The actions are set to LowPriority so they don't show on the systemtray header, which is virtually the current situation.

Now, no un-updated hamburger menu will appear when opening the context menu with a right-click.

Fixes this
mediacontroller-actions-before