Modify MWI subscription setting on run-time and add callback for the subscription state change

[nanang]

Modifiable settings to be implemented:

• pjsua_acc_config.mwi_expires: subscription expiration interval (compile-time default & run-time)
• pjsua_acc_config.mwi_enabled: to enable or disable MWI subscription. It can be updated via pjsua_acc_modify() to start or stop MWI subscription.

When there is a MWI subscription state change, pjsua callback on_mwi_state() will be called.

MWI subscription will be terminated when deleting the associated account.

Thanks Peter Koletzki and Matynka Bartosz for the feedbacks.

[nanang]

In 4172:

Close #1540:

• added pjsua_acc_config.mwi_expires, also compile-time macro PJSIP_MWI_DEFAULT_EXPIRES
• updated pjsua_acc_modify() to update MWI subscription when mwi_expires & mwi_enabled of pjsua_acc_config is modified

[ming]

In 4180:

Re #1540: Modify MWI subscription setting on run-time

• Add subscription state callback
• Terminate subscription when deleting acc

[ming]

In 4186:

Re #1540: Fixed assertion when terminating MWI subscription