Use isDownloading expose from model instead of downloadingImageIndex property
from GagManager
Add CustomCountBubble for votesCount and commentsCount UI
Use 9GAG comment system at comments page
Changes to the backend coding design

* Remove GagManager, move most of the functionality to GagModel
* Add GagBookManager which hold an instance of NetworkManager and
AppSettings to be used by GagModel
* Remove section from AppSettings and no longer remember section choosed
after exit
* Move section enum to GagModel
* GagModel will automatically refresh in C++ side when component
completed
Fix unable to download GIFs
Bump version to 0.8.3