Port KKeySequenceItem to QQC2

Summary:
This implicitly fixes a bug where space is used in the shortcut item.
Previously as QQC1 button was made up of multiple objects the key
handling would be handled by an internal object before our filters.

In QQC2 it's all one item, so our Keys handler takes precendence.

Test Plan:
Added a test file
Set the button to control+space, alt+4
Set the button to alt+space (this correctly showed a warning about a conflict)
Confirmed tooltip worked correctly
Used space to activate the button when we weren't recording. This someone still worked..

Reviewers: #plasma, davidre, ngraham

Reviewed By: ngraham

Subscribers: ngraham, cblack, broulik, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D29292