Make Zoom shortcuts consistent with OS shortcuts

[olekscode]

In Mac (and I think also in other OS, as well as in browsers), there are standard keyboard shortcuts for zooming:

• Cmd + to zoom in
• Cmd - to zoom out
• Cmd 0 to restore the default zoom

In Roassal, we use

• I to zoom in
• O to zoom out
• <space> to restore the default zoom

I think, we should be more consistent with OS, otherwise, people would find Roassal interactions unnatural

[akevalion]

moved to pharo-graphics/Roassal