#168 ALT GR as SYMBOL SHIFT

[efa]

mapping of "Alt Gr" key to SYMBOL SHIFT. At least with an option.
That key is in the same position as the original SYMBOL SHIFT in rubber keybord, so seems natural to many users use that instead of LEFT ALT key.
I think both ALT key should work as SYMBOL SHIFT, like both SHIFT work as CAPS SHIFT.
Other emulators use ALT GR as SYMBOL SHIFT

[efa]

PS. I saw now that on Windows is already working ALT GR.
Wont work for me on Linux, with MATE desktop manager and X.

Strangely on Windows LEFT ALT won't work, while work on Linux.