Add edit mode menu item to desktop widget context menu

Summary:

• much improves discoverability of the ability to move added widgets without clicking and holding (I have seen people having trouble figuring it out)
• makes it more consistent with panel widget context menus
• allows easier access to the edit mode when the desktop is full of widgets

Test Plan: Open the context menu of a widget on the desktop.

Reviewers: Plasma, VDG, mart, ngraham, davidedmundson

Reviewed By: Plasma, VDG, ngraham, davidedmundson

Subscribers: davidedmundson, ngraham, broulik, mart, VDG, kde-frameworks-devel, Plasma

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26043