remove unused function
don't change focus when phone is in use

Also, rework a few cases where the focus was changed in a non-optimal
way.
clear incoming calls selection when focusing on waiting calls
call focusOnIncomingCalls on first queue entry update

Without it, if your phone was already ringing from a call from the
switchboard queue before you logged with your xivo client, the current
call widget would not be in the correct state.

An another way to do it would be to check the number of queue entry
during the switchboard xlet initialization, but the model doesn't
contain this information yet.
change how current call buttons state changes

It should now reflect more closely the state of the phone.

This should also fix #5265.
remove m_first_queue_entry member
use "this->" in a more systematic way
Merge remote-tracking branch 'origin/5250-switchboard-xfer-focus'