Support Push Notification in iOS sample app

[sauwming]

Provide an example on how to implement Push Notification using PushKit and CallKit frameworks in ipjsua sample app.

Also in this PR:
Add sample on how to call PJSIP functions from a PJLIB registered thread via timer.

[nanangizz]

Is two/more incoming calls scenario handled, e.g:

• concurrently, user may want to choose which call to answer, or
• another incoming call in the middle of an active call.

[sauwming]

Ah right, currently it's not handled yet.
I will add this to the implementation.

[sauwming]

Is two/more incoming calls scenario handled, e.g:

• concurrently, user may want to choose which call to answer, or
• another incoming call in the middle of an active call.

Done.

[nanangizz]

Looks okay to me.

Btw, the patch seems to also add sample code of how to call PJSIP functions from a PJLIB registered thread via timer, perhaps add desc about this too?