[core] Added support to get a list of all non-special categories.

Except for `Uncategorized`.
[sailfish] Show only non-special categories in AddSubscription

Except for Uncategorized.

Regards #134
[sailfish][Refactoring] AddSubscription; inlined ComboBoxList
[sailfish][Refactoring] Extracted function
Merge branch 'add-subscription' of git://github.com/michael-k/ttrss into michael-k-add-subscription
[core] reuse the category model for feed subscriptions