Add support for multi-key button pres bindings

[035059]

Hi there,

This is my first time contributing to an open-source project, so apologies if there is some etiquette I'm not familiar with.

I wanted to be able to bind buttons to key combinations (eg. Shift+1) to get support for the view controls in Onshape without modifying the keybinds.

This extends the button-press handing to allow for the use of up to 8 keys being bound to a single button, without breaking existing configs. I also added some logic to the config parsing, to give feedback to the user if their config is malformed.

Example /etc/spnavrc line:
kbmap30 = Shift_L+7

The config I set up for Onshape is working properly, but I haven't done extensive testing for edge-cases.

[jtsiomb]

Thank you, this looks great!

The only problem is that the protocol requests (SCFG_KBMAP/GCFG_KBMAP) don't support multiple keys. I added a TODO comment to see if I can find a way to add that capability in a backwards-compatible way (otherwise we'll have to introduce new requests).

The only thing I'd do different would be to terminate the key sequence with a 0, instead of having a separate count array. But it's not important enough to change it, so I left that part unchanged.