feat: Granular control over which types of apps are themed

Feature description

Currently, Gradience is able to theme natively installed distro packages and has toggles to enable theming of GTK3 and GTK4 flatpak applications.

I think it would be useful to make the option for which apps are themed more granular. For example, there should be toggles to enable theming of:

natively installed GTK3 apps
natively installed GTK4 apps
flatpak installed GTK3 apps
flatpak installed GTK4 apps
(once supported), snap installed GTK3 apps
(one supported), snap installed GTK4 apps
(once supported), gnome-shell

Additional Context

The reason I suggest it to be this way is due to the way theming works on Ubuntu.

Ubuntu relies on a patched version of libadwaita to theme GTK4/libadwaita apps with Yaru. This patched version of libadwaita works for natively installed apps and snap installed apps. However, this patched version of libadwaita is not available for flatpak apps.

So I would want to use Gradience to just theme GTK4/libadwaita using the Gradience yaru preset. However, I would not want Gradience to theme native GTK3/4, snap GTK3/4, or flatpak GTK3 because the Yaru theme already works with all of those formats and generally looks better than the Gradience Yaru preset.

Did you read the Code of Conduct?

[X] I have read the Code of Conduct.

GradienceTeam / Gradience