key bindings for Deselect (Shift+Ctrl+A) and SaveAs

Issue 88
https://github.com/callaa/Drawpile/issues/88

Please note I've used the QT built-in keybinding for Save As and based on the documentation I cannot assume this keybinding will appear on Windows but it seemed preferable to hardcoding it. 
http://qt.developpez.com/doc/4.6/qkeysequence/#standardkey-enum