[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
Fixed leftover
Update all views when changing showAll

We cannot update all views at once, because this happens:
```
file:///usr/share/harbour-ttrss-cnlpete/qml/models/FeedModel.qml:51:
TypeError: Cannot read property 'categoryId' of null
```

Tested on sailfish
Closes #142
[sailfish] Cleaned up (DRY)
[core] reuse the category model for feed subscriptions
Merge branch 'fixShowAll' of git://github.com/michael-k/ttrss into michael-k-fixShowAll
only redisplay categories with showall flag on updateView(), regards #142