Warn when overwriting a configuration rather than just adding new entries

[magnus-ISU]

I'm thinking that if any exist, it should create a popup listing all the configurations that will be overwritten (for example, if brave is set to open x-scheme-handler/https and then I set firefox for all defaults, then it should show up

Warning: Some previously-configured settings will be overwritten:
Brave currently opens x-scheme-handler/http
Brave currently opens x-scheme-handler/https

[ Overwrite settings ]   [ Only add non-conflicting rules ]   [ Cancel all changes ]

Those button names suck, the middle one especially is way too long (but I would rather err on the verbose side to the non-verbose ok vs cancel crappy design you see everywhere), if anyone has suggestions to shorten them.

I will probably open a pull request here in a couple days (or maybe hours idk, shouldn't be too hard to do I think)