File Dialogs adjustments (v2)
Created by: Krakean
Made adjustments to open/save file dialogs according to notes from Akien (see comment https://github.com/godotengine/godot/pull/13252#issuecomment-347045849), plus a bit more:
- Added "go to parent folder" (^) button to Save a File dialog.
- Tree.cpp: "nothing_selected" signal has been re-made
- Fixed issue in Project Export dialog: MODE_SAVE_FILE wasn't set when you click "Export".
See commit message for all details.