Allow changing order of open document tabs

[sean-h]

Allows a user to reorder open document tabs; closes #407 .

[bjorn]

Hmm, there was some reason that I didn't enable this like that. Are you sure everything is working correctly with that change?

[stefanbeller]

If I recall correctly, the either the saving of the documents or the restoring order when restarting tiled was a problem in the discussion we had years ago.

[sean-h]

I didn't notice that it wasn't restoring the modified tab order when restarting. I added a method to reset the ordering of the open documents list. Qt doesn't seem to have any way to respond to a tab reorder event when using QTabWidget so I set it to update whenever the tabs may have been changed since last access (closing the program, closing a tab, switching tabs).

[bjorn]

Hmm, this looks a little brute-force to me. Could you try to make a QTabWidget subclass that forwards the tabMoved signal from its QTabBar, and use that? The subclass is necessary because the QTabWidget::tabBar() accessor is protected.

[bjorn]

Should be able to use mDocuments.move(from, to).

[bjorn]

Thanks, @sean-h!