(2/3) Move "Configure..." from save menu into its own button

Summary:
Part 2 of 3 for T7841: Revamp buttons on the bottom to solve various usability issues
Depends on D10283

BUG: 375965
FIXED-IN: 18.04

Nobody can ever find Spectacle's Configure window because the menu item to invoke is hidden in the Save menu, oddly enough. Once D10283: (1/3) Remove unnecessary Discard/Quit button lands, there'll be room for another button. So, we move the "Configure..." item out of the Save menu and into its own button.

Test Plan:
Tested in KDE Neon:

• "Save" split button no longer has a "Preferences" item
• Button works and opens the Configure Spectacle dialog
• Button responds to standard ctrl+shift+, keyboard shortcut
• Button has standard "configure" icon

Reviewers: Spectacle, alexeymin, rkflx

Reviewed By: Spectacle, alexeymin, rkflx

Subscribers: alexeymin, rkflx

Differential Revision: https://phabricator.kde.org/D10289