Media count manipulation in call basis (thanks Bogdan Krakowski for the suggestion)

[nanang]

Currently, media count setting is configurable via account setting only, which not really ideal for application, e.g: need to modify the account setting just for enabling video in an audio only session. So, media count manipulation should be done per call basis rather than per account basis.

[nanang]

(In [3891]) Re #1419, implement media count manipulation per call basis:

• moved the media count setting from account setting to call setting
• introduced pjsua_call_setting, to be used by pjsua_call_make_call() and some new APIs: pjsua_call_answer2(), pjsua_call_reinvite2(), pjsua_call_update2()

[nanang]

(In [3903]) Re #1419: updated call transfer to allow both the transferee and the transfer destination to update the current call setting:

• for transferee (attended & unattended): via new PJSUA-LIB callback on_call_transfer_request2()
• for transfer destination (attended only): via new PJSUA-LIB callback on_call_replace_request2()

[bennylp]

(In [3925]) More re #1419: changed field names from video_count to vid_cnt and audio_cnt to aud_cnt for better consistency