Tag hierarchy

[RvanderLaan]

Closes #3 (sort of)
Replaces the list of tags with a tree of collections of tags.
The basic functionality works (adding/renaming/removing tags and collections).
Still todo:

• Reordering using drag and drop ( Manually order tags #24 )
• UI improvements (it doesn't fit after 3 or 4 nested collections)
• Setting the initial name of tags and collections; currently they are initialized to 'New tag/collection'

[RvanderLaan]

This feature is finished (for the most part), so it can be merged.
The TagCollection has been added as a new entity, which contains sub-collections and tags.
The only things that I'm not entirely happy about is:

1. The hierarchy of collections and tags passed into Blueprints Tree component is generated on the fly from the collections stored in the TagCollectionStore. Usually this hierarchy is stored in a state somewhere instead, but I had some doubts whether that would make sense in our case (more comments here).
2. Collections can be dragged onto other collections to move them into that collection, but they cannot be dragged around to change their order as is possible with tags. I'm not sure how other applications achieve this while remaining intuitive to use

[hummingly]

@RvanderLaan For some reason a lot of collections are permanently selected and the search has two search fields for tags.

[RvanderLaan]

Thanks for merging! I was just about to do that.
Yeah, I'll fix that. It marks collections as selected when all of its tags are selected, but apparently also when it has no tags. But I'm not really happy with the behavior of selecting items in the tree in general.
I can't find any examples where you can select multiple items in a tree, for both leaves and inner items.

The two search fields were supposed to be for including and excluding, I'll add proper labels to those

[hummingly]

Can we merge?

[RvanderLaan]

Yes! The honor is yours