Show default key assignments

[xarx00]

Some keys appear unassigned, while in fact they have a predefined functionality in the editor.

E.g.

CTRL+Y, CTRL+Z, CTRL+X, CTRL+C, CTRL+V
CTRL+INS, SH+INS,...
SH+LEFT, SH+CTRL+LEFT, SH+ALT+LEFT,...
CTRL+-, CTRL+= (zoom)
etc.

Would it be possible to list them too? When I'm looking for an available key combination, these keys seem to be free to use, while they are not.

I suggest to create a switch for this. The list of default key combination may be platform dependent.

Perhaps related to #5.