Subscribing and unsubscribing to subreddits.

Users can now subscribe and unsubscribe to subreddits. This
functionality still needs to be added to subreddit list in the search
view.
Q10 support and UI optimizations for dark theme.
QML optimizations with Colors.
"Goto subreddit" feature added. Users can go to subreddit directly by
inputing its url.
Comment voting, and "to top/bottom" actions for comment list.
Pages are destroyed when popping throughout the application.