Improve UX of drive letters

Namely, move the drive dropdown to just the left of the path text box and don't include the former
in the latter.

This improves the UX on Windows.

In the UNIX case, since its concept of drives is (ab)used to provide shortcuts to useful paths, its
dropdown is kept at the original location.

core/os/dir_access.cpp

@@ -66,6 +66,11 @@ int DirAccess::get_current_drive() {
 	return 0;
 }
 
+bool DirAccess::drives_are_shortcuts() {
+
+	return false;
+}
+
 static Error _erase_recursive(DirAccess *da) {
 
 	List<String> dirs;