Add hotkey to toggle between the two most recently selected layers

Although this generally seems to work fine, I have noticed two minor problems that I'm not sure how to solve:

• Using the hotkey after reordering layers will cause the wrong layer to be selected. This is because activateNodeImpl() gets called when a layer is moved around in the stack.
• Bug 373741 ("Drawing while switching between layers with PgUp/PgDown results in a layer acting like it's in a locked group") applies and is easier to reproduce (at step 2, hold the hotkey instead of alternating page-up and page-down).

Differential Revision: https://phabricator.kde.org/D3750
Ref T1734