SelectCategory and AddCategory activities refreshed

This branch:
* replaces (in post editor) category list by category buttons to quickly remove categories from a post
* adds hierarchy representation in the SelectCategories and AddCategory (parent selection) activities
* fixes #213: Adding a new category remove the selection on the categories screen.
* fixes #258: Adding a category results in an unsorted category list.
* adds a new DB version (v.18): add parent_id in categories table (local db) with migration code

Notes:
* FlowLayout is largely inspired by this video: http://www.parleys.com/play/514892280364bc17fc56c0e2/chapter38
* I don't know where to put the PSD "source" file for the new icon