Use move to trash in the project manager instead of immediate delete

This MR implements the platform-specific calls to move files and directories to the trash bin/recycle bin instead of immediately deleting them when the user clicks on the delete menu item in the project manager. The added benefit for doing it this way is we no longer must recursively delete files in directories, we can simply pass the entire directory in at once and move the entire structure to the trash (so the internal structure is also preserved in the trash).

This adds the previously-announced linux dependency on GTK3 to the CMake build, so any builds without the dev packages for it will fail once this is merged.

Fixes #2499 (closed)