make InputMethod immutable

previously enabled and active were writable properties which complicated
abstraction inside application sandboxes. to solve this problem we no
longer consider them writable in KF6 and consider their writability
deprecated in KF5.

active is controllable through Qt's IM API and enabled shouldn't really
be touched by random applications anyway

this then allows us to eventually back them using the XDG Settings
portal instead of talking to KWin directly.

CCBUG: 466252