Track the actual pressed keys in ModifierOnlyShortcuts

Summary:
With this change the ModifierOnlyShortcut starts to track the actual
pressed keys instead using a counter of combined pressed keys.

This should help for the cases that we get unsynced key codes.
E.g. if we get two key presses for the same key and only one release
we don't get out of sync. Similar if we get a key release for a key
which wasn't pressed, we don't go out of sync.

Test Plan: Auto test still passes

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D4617