Expand GTK (and Qt) Support

This is to address several issues Discord Client has on Linux distros:

1 - No native style blending.
This is something that was already fixed on macOS by including Client Side Decorations matching the general macOS theme, which is the left side buttons for close, minimize and maximize. It could be easily fixed by expanding the GTK support to have CSD matching the selected theme, just like how it uses a GTK window for upload popup. If you are using GTK already, why not use CSD or at least give the users the option to enable it natively and have a similar experience as Firefox with them or even macOS version of Discord Client? As of right now, Discord looks like an alien with its own theme and calling Server Side Decorations which never matches Discord's theme.

2 - No Qt support.
You know, GTK is not the only Widget Style out there, even less the only one used for Desktop Environments. One of the most used DEs out there, KDE Plasma, is left out without any kind of integration with Discord, except only for the appmenu export for options which are completely dispensable. Would have been great if the client detected which desktop environment it is on, to swap the upload popup, for example, to use Qt instead of GTK. Not only this but also bring better desktop integration to allow drag and drop, for example.

Those two issues can be easily fixed and I honestly hope they do. If you are unsure how to do that, look for Community patches like Discord Adapta Nokto to know how at least make Discord blend better with simple CSS changes. Now that Discord is on Linux, why not adapt to it?