dolphinQt: fix Qt6 deprecation warnings

Avoid map/set double lookups

Fix some common anti-patterns with these data structures.

- You can dereference the iterator returned by `find` to access the
  underlying value directly, without an extra `operator[]`/`at`.
- Rather than checking for an element before insertion/deletion, you can
  just do the operation and if needed check the return value to
  determine if the insertion/deletion succeeded.

ClickBlurLabel: Create a label which is blurred until it is clicked

This is particularly useful for people who stream their desktop and don't want to accidentally leak their IP or room.

DolphinQt: Replace widespread SetQWidgetWindowDecorations calls with an event filter.

Core & UnitTests: Make overriding explicit and remove redundant virtual specifiers on overriding destructors