Mastodon support

[Andrea Scarpino]

Hi,

I'm starting to implement Mastodon[1] support in Choqok.

At the moment I implemented basic stuff like:
- authentication via OAuth2
- toots (posts are named so in Mastodon)
- (un)favourite button
- toots deletion

Since it uses OAuth2 I'm using the QtNetworkAuth module and since that branch 
hasn't been merged in master yet (and also because my work is really at the 
beginning) I pushed everything to a 'mastodon' branch.

Note: authentication workflow is broken when you use a custom browser and you 
are also using Plasma (technically when you set XDG_CURRENT_DESKTOP=KDE). 
That's because of a bug in kde-open5[2] that I'm investigating.

[1] https://mastodon.social
[2] https://bugs.kde.org/show_bug.cgi?id=364662

-- 
Andrea