Cancelling 'save as' dialogue should not quit application when saving unsaved work

[gavanderhoorn]

As noted in #74, the current implementation quits the application whenever the user presses cancel on the save as dialogue that is opened when choosing to save unsaved work.

Expected behaviour is for the user to be taken back to the application window, not exiting.

[AndyZe]

@gavanderhoorn @phvass Can you guys assign this one to me?

[AndyZe]

save as works fine for me (on the m1_merge_candidate branch)