[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: {F7877561} {F7877562}

After: {F7877563} {F7877564}

Reviewers: #vdg, #breeze, ndavis

Reviewed By: #vdg, #breeze, ndavis

Subscribers: ndavis, plasma-devel

Tags: #plasma

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