more abstract interface to speeddials

Trying to hide internal structure so that it will be easier to split
into multiple files.
rename speeddial_group to sccp_speeddials
a bit more abstract interface to lines

So that it will be easier to split into multiple files...
rename reset_on_no_subchan

Also structure it so that it would still works in the multi-lines case.
remove simple speeddial accessor

The structure won't be completly opaque, so this is not necessary.
remove function pointers for device state