Have KHamburgerMenu's shortcut open a menu

For accessibility we need KHamburgerMenu to open a menu no matter
which state the application is in. Pressing the shortcut will:
- trigger/open a hamburger button if one is available
- otherwise trigger/open the menu bar if that one is available
- otherwise open the hamburger menu at the mouse cursor position

Keyboard shortcuts don't get triggered if the connected action is
set to invisible. Before this commit, KHamburgerMenu set itself
invisible only to hide the hamburger button whenever the menu bar
is visible. This commit changes this so the KHamburgerMenu action
is always visible. Therefore the keyboard shortcut always works.
Instead, the buttons belonging to the KHamburgerMenu action are
hidden whenever we would have hidden the action.

A new test is added for the new behaviour.
The previous tests are adapted to work even though the logic for
visually hiding the hamburger buttons has changed.