Support modifier-only-shortcuts when capslock is on

Summary:
So far we didn't trigger modifier-only-shortcuts when capslock was
enabled. In fact we even ensured that the shortcuts did not trigger.
This seems not to be what our users expect. Meta should still trigger
if capslock is on.

This change modifies the logic to determine which modifier is currently
pressed by using the modifiersRelevantForGlobalShortcuts. The difference
to the "normal" modifiers is that this excludes capslock from modifiers
and excludes consumed modifiers. The latter is not really relevant as
modifier-only-shortcuts do not trigger if multiple keys are pressed,
which is required to have a modifier consumed.

BUG: 375355
FIXED-IN: 5.8.6

Test Plan: Only with adjusted autotest

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D4241