The tool menu has several keyboard shortcuts to speed up your workflow:

V – Selection tool

B – Brush tool

E –  Eraser tool

H – Move tool

Holding down spacebar will temporarily enable the move tool so you can drag around the level to change position

The File menu also has shortcuts:

⌘+O – Open games

⌘+S – Save game

⌘+Shift+S – Save a copy of the game

Note: Windows users should hold CTRL instead of ⌘