feat(#2989)!: hover for all modules

[haug1]

I recognize that this change may break or change many existing style.css configurations due to the new buttons and the default configuration containing a general css rule for buttons.
I've just done the minimal job with this approach of implementing the feature. If this approach is acceptable even with breaking changes to existing configurations, I can continue finishing this up with some guiding feedback.

If this is not an acceptable approach, I've been thinking about a different approach of just adding a ".hover" class to the GTK label instead. I haven't really explored that possibility yet and I'm very new to GTK and C++, so I'm not sure about the details in that approach, but it sounds very doable and probably not hard to figure out how to implement. This approach would likely not breaking any existing configurations.

[haug1]

#1792
This approach seems less problematic than the one in #1120, but we're probably gonna see similar breaking changes to style.css configuration as mentioned here. A big challenge for implementing hover functionality seems to be that only GtkButtons offer this :hover in GTK CSS land. So we're either never going to have "true" CSS :hover functionality or we have to implement a breaking change.

[haug1]

@Alexays
Is it interesting? Could you please take some time to give feedback on this? How can we got some feature like this merged?
I'm determined to add this hover feature, but maybe this is not the right way because backwards-compatibility. Do you have any thoughts about the alternate solution hooking up GTK mouse enter/leave events and adding/removing the .hover class?