This commit adds a preview to the properties window for transitions.
The preview will play back the transition at the global transition
duration or the transitions fixed duration, between two private scenes
with an A and B label, and different background colors.
When clicking the close button with unsaved changes in a source properties
dialog closeEvent and reject are called, which both call ConfirmQuit.
This fixes it by setting the acceptClicked flag correctly in ConfirmQuit
Use the Qt standard buttons in the source properties dialog to make the
UI consistent. Also remove the right alignment of the button box to
allow Qt to align standard buttons on the left side of the dialog.
Closesjp9000/obs-studio#1013