GDScript editor goodies

Added:
- Clone Down: Duplicates the current line. (Cmd/Ctrl+B)
- Move Line Up/Down: Move current line up or down (Cmd/Ctrl+Up/Down Arrows)
- Indent Left/Right: Indent/Unindent Line (Cmd/Ctrl+Left/Right Arrows)
- Toggle Comment: Comment a Line or Selected lines (Cmd/Ctrl+Slash)
- Some functions like: insert_at() and set_line()