RFC: stylus tool id aliases

[whot]

Let's add a line in the form Aliases=0x1234;0xabcd to libwacom.stylus so we can easily duplicate tools that are identical but have had (or will have) multiple tool ids over time.

Note: this needs to be compatible with the new vid:pid notation: #759

[whot]

After some private discussion with @Pinglinux this si probably best solved by this approach:

[0x100802]
Name=Art Pen
PairedStylusIds=0x100804
... other fields...

[0x802]
AliasOf=0x100802
PairedStylusIds=0x804

In other words, we still have entries for styli but declare those to be an alias of another stylus and only define the fields where they override the original stylus' setting.

The originally suggested Aliases=0x802 doesn't work too well for the PairedStylusIds settings which also needs an update for aliased styli.