Remove linux-only guard in drag-drop implementation

This commit:
- Takes out an `ifdef` that kept the important bits from actually happening on non-linux platforms
- Adds `Poco/URI.h` to the included files for all platforms

I *haven't* tested this on windows, and I also don't know why there was an `ifdef` guard in the first place, so this could use a +1 from @arturoc at least :) (due to fe347fea16bf349544add7c028258e7fe3314cd0 )

Fixes #3059