[3.x] Backport exposing EditorResourcePicker and using it in the Inspector

[YuriSizov]

This backport includes:

1. Exposing EditorResourcePicker and EditorScriptPicker (Add EditorResourcePicker control based on the Inspector editor for Resources #47260, with Make EditorFileDialog only created on demand in EditorResourcePicker #49068 bugfix).
2. Use new controls in the editor's Inspector dock (Use EditorResourcePicker in the Inspector #48854).

Should be more or less the same as the master version. I've also noticed that there is no code for auto-converting some data types into their appropriate resources, the code is present in master. I saw no downside from including it, though I had to omit FontType -> Font conversion due to fonts being very different in 3.x.

All changes are split into individual commits related to each backported PR.

[YuriSizov]

I have no idea what's wrong with macOS editor builds here. The errors highlighted make no sense as these are internal parts that I don't touch. So there is probably something else I didn't consider, specific to 3.x.

[YuriSizov]

Okay, fixed macOS problems (we don't use overrides in 3.x). Should be good to go.

[akien-mga]

Thanks!