[applets/systemtray] Support high priority actions in the header

Applets can now mark an action as high priority by setting their priority value to 256 (QAction::HighPriority), and this action will be always shown on the toolbar instead of getting included into the hamburger menu.

This helps keeping the most important action easily accessible even after adding new actions, solving some design issues.

Some examples
Battery monitor
Notifications (!2242 (closed))

The volume applet will probably benefit too (until its config options are merged into the kcm).

There's some code repetition now, but I'd rather leave a refactor for a different MR.