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