qt: add 'expert' preferences mode

I'm excited to finally submit, 'expert' prefs mode, originally made back in 2019. (For Qt only, I'm not in the best position to bring it to MacOS).

This new 3rd mode gives a table listing all options, similar to and inspired by the about:config interface in Firefox.

Unique benefits:

• It highlights which options have been modified from default state.
• It allows selective resetting of individual options to default values.

See the last commit log for more details.

Fixes #18607 (closed).

Screenshot example of the new view and the context menu:

Screenshot example of the editing dialog. The control used in the dialog depends upon the option type, using the exact same ones as in the advanced view.

Some follow-up work plans:

• I intend to bring similar enhancements to the hotkey editor (context menu with reset option; non-default highlighting).
• I plan to explore a bit of a visual redesign for the overall preferences interface (and maybe effects too).
• I plan to unify the three interfaces such that a change in one is reflected in the others and saving will save changes made in all views.