Explain hitting 'Cancel' on keymap-changer.

[triantares]

When the 'choose Variant' window comes up, some newcomers can be baffled as they might not know what variant (if at all) they've got. Especially those with standard keymaps never realized those variants even existed.
The tool itself is fairly unforgiving when trying out for 'variants' and an additional text should be added to explain that hitting 'Cancel' will simply keep the standard (i.e no variant) and exit the tool.

Optionally there could be an extra entry like 'None, keep the standard' but that would more work than it's worth IMO.