more generic feature modules

[totaam]

Continuing #1838, #1778, #2578, #1836, #4307, #1861
And very helpful for implementing #3981, #1942

Each feature module should declare its own private prefix, which cab be used for:

• capabilities
• info
• packet handlers

[totaam]

still TODO:

• split pings and info from network state
• split keyboard and pointer (but some functions must be shared for modifiers..)
• file + print
• window and display mixins are a mess
• dispatch packets via mixin prefix adapter?