Open context menu on Shift+F10

Users who can't use a mouse pointer or other pointing device rely
on the keyboard to open context menus. However, many keyboards
these days do not have a Menu key. Therefore there is no way to
open context menus by only using a keyboard. This is a big issue
for accessibility.

Everyone else has already decided that context menus should be
triggered by Shift+F10. This is already recommended by the Web
Accessibility Initiative, Microsoft, and implemented in Chrome/ium
and Firefox.

This commit posts a QContextMenuEvent whenever a Shift+F10 key
event makes it to KMainWindow. This only happens if this key
combination isn't already consumed before that for example because
it is assigned to an action in application code.

Some actions do not react correctly to QContextMenuEvents because
they might be hard-coded to only react to right-clicks or Menu key
presses specifically. Those will need to be adapted later for this
to work as expected everywhere.

This commit will have no effec...