symbols: Allow levels with different keysyms and actions counts

[wismill]

Contrary to groups, there is no reason for levels to restrict the same count of keysyms and actions. See the related discussion with @mahkoh.

This will allow better support for the use case Modifier + Group change, since we only want to communicate the modifier keysym to app, not the group keysym.

Also always normalize the multi-keysyms/actions lists by dropping NoSymbol and NoAction.

Fixes #619

TODO:

• Modify xkbcomp so that it ignores multi-actions per level, as it already ignores multi-keysyms.

[Jules-Bertholet]

Do we really want to drop NoAction()? It’s really somewhat of a misnomer, as it does perform an action—breaking latches.

[wismill]

@Jules-Bertholet I though about introducing VoidAction(), to mirror NoSymbol/VoidSymbol. Because currently we also do not have an easy way to erase previous explicit actions when using RMLVO:

• NoAction() does not overwrite the previous action.
• replace key works only in the same file or after a child file.
• SetControls() kind of work if you want to break latches. But it’s ugly.

The no/void qualifiers are not very clear, but at least we would be consistent with the keysyms.