[GTK3] Improve Electron menubar legibility

Summary:
Electron apps using a menubar are now more legible. This has the side effect of
forcing background tabs in Chrome/ium to not have an inactive appearance, as window > menubar
provides the styling for both Electron's menubar and Chrome/ium's tabs.

BUG: 413915
FIXED-IN: 5.18

Test Plan:
Before:

After:

Reviewers: VDG, Breeze, ndavis

Reviewed By: VDG, Breeze, ndavis

Subscribers: ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26587