WIP: Explicit object order with actions to change it

The raise/lower actions are currently only available in the object
selection context menu.

TODO: Make this new object drawing order behavior optional.

Sponsored-by: Ben Wales
Unduplicated the duplicate and remove objects actions

Also shared the implementation of moveObjectsToGroup by moving that
function into MapDocument.
Implemented moving objects with arrow keys

Closes #342

Sponsored-by: Ben Wales
Introduced shortcuts for raise/lower actions

* PageUp => raise
* PageDown => lower
* Home => raiseToTop
* End => lowerToBottom

Previously, PageUp and PageDown changed the selected layer. Those have
been moved to Ctrl+PageUp and Ctrl+PageDown.

Those shortcuts in turn used to change the currently selected map. This
is also possible with Ctrl+Tab and Ctrl+Shift+Tab, but to make up for the
lost shortcuts, Alt+Left and Alt+Right were added.