Replace category list in EditPost by category buttons

- click on category remove it from the post
- introduce a new layout: FlowLayout
- some refactoring (don't use JSONArray to manipulate data)
New database version (v18) atlering categories table to add parent_id
add new icon: level_indicator used in select category view
Refresh select category view.

- Rewrite a part of category managing
- Display category hierarchy
- this also fix #213: Adding a new category remove the selection on the categories screen
- and fix #258: Adding a Category results in an unsorted category list
Display category hierarchy in "parent" spinner of "AddCategory" view

- also merge Category / CategoryNode / CategoryLevel into on CategoryNode class
New icons on delete and select category buttons
Remove "Selected Categories:" label
Remove useless import
Reset _Uncategorized_ button padding
make ic_level_indicator.png icon thinner
fix #213: Adding a new category remove the selection on the categories screen
Remove auto addition of special "Uncategorized" category
Add a category label and change the "Category +" button behavior
Merge pull request #21 from maxme/new-categories

SelectCategory and AddCategory activities refreshed, props @maxme.