Add edit mode menu item to desktop widget context menu
- 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.
Differential Revision: https://phabricator.kde.org/D26043