- Add the shortcut editor: still being implemented

- RotoGui: don't do the action if there's no point to apply it to
- Fix the GuiMacro bug introduced in
https://github.com/MrKepzie/Natron/commit/c58083ed00d1898d0c7efc14bc79ce
9c83efaa43
- Fix the matchesMouseShortcut function to have the following behaviour:

Note that the default configuration of Apple's X11 (see X11
menu:Preferences:Input tab) is to "emulate three button mouse". This
sets a single button mouse or laptop trackpad to behave as follows:
X11 left mouse button click
X11 middle mouse button Option-click
X11 right mouse button Command-click
(Command=clover=apple key)