Commit-message Editor-window is missing OK button, opens on other monitor

[goran-w]

Follow-up to #2096 - there is no longer any button (OK or similar) visible at the bottom of the (Merge-)commit message Editor window? (The window can be closed with the X close-button in its upper right corner, but it's unclear to the user whether this is supposed to mean "OK" or "Cancel"/"Abort"...)

Also, related to #2100 - this Editor window appears on another (the primary) monitor, but it would be much better if this (sub-)window appeared at the center of the current main window.