Fixes#7146
Normally, Qt and CEF each run their own version of GTK, often causing
conflicts. This commit introduces a build of CEF where GTK has been
disabled using `use_gtk=false` as we do not require the Chrome runtime.
This is an experiment and may end up being the safest long-term solution.
Note: This does introduce potential quirks/incompatibilities, such as
browser sources no longer being able to display alert() dialogs, and
potentially file browser dialogs not working from CEF.
This is better than the alternative. If necessary, such dialogs can be
reimplemented using Qt at a later date, and would likely provide a more
consistent user experience.
0e7f69ee31