Allow keyboard focus for toolbar actions

Visually-impaired users need to be able to trigger common actions
easily. Applications have a curated list of those on the toolbar,
but prior to this commit most of these buttons in most
applications would not accept keyboard focus in any way,
effectively prohibiting blind users from discovering or triggering
this curated list of actions.

If actions are only available from the toolbar, it could even be
possible that users who only use the keyboard would have no way of
triggering those actions at all. However, KDE design guidelines
typically encourage that those actions are also available through
a menu bar.

This commit fixes the above. Now all buttons on the toolbar will
get keyboard focus sooner or later when a user presses the Tab key
repeatedly.

As a side-effect this also means that the context menu of those
actions can also be opened by keyboard, because the Menu key will
open the context menu when such an action has keyboard focus. This
is nice because there can be advanced functionality in there. For
example in Dolphin: In the context menu of the "Go Back" action
there are entries to directly jump to a previously visited
location.