Add support to select multiple contacts for MMS
CC @guido.gunther @mohammed.sadiq @tobias.bernard
EDIT: All of the issues were fixed.
This MR add support for including multiple contacts to make a Group MMS.
To test: Set has_mms to TRUE.
https://source.puri.sm/kop316/chatty/-/blob/wip/group_mms_dialog/src/chatty-window.c#L425
Some Screenshots (All numbers are fictitious):
