Fix ambiguous shortcut warning when pressing save shortcuts
When Remember last used save mode is enabled, pressing
Ctrl+S or Ctrl+⇧+S repeatedly would result in a
ambiguous shortcut warning dialog.
This is because on every save the action was re-added to the dropdown
button, resulting in multiple shortcuts. It should be enough to set the
actions once, and then only swap the default action.
Press Ctrl+S twice, no warning dialog is shown anymore.
Both Save options still work as before and the last-used button is
set as the default.
Reviewers: Spectacle, ngraham
Reviewed By: Spectacle, ngraham
Maniphest Tasks: T7841
Differential Revision: https://phabricator.kde.org/D10469