Grab the keyboard when KeySequenceItem is recording

Summary:
Otherwise assigning a key combination which is currently bound to a global
shortcut action will trigger the global shortcut.

Test Plan:
Try to assign a global shortcut to KKeySequenceItem. The global shortcut is
not triggered - instead the steal shortcut prompt is displayed.

Reviewers: Frameworks, davidedmundson, apol

Reviewed By: davidedmundson, apol

Subscribers: apol, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24736