Support custom root categories

[Xiphe]

Given I have two budgets (private and freelancing)
And I want both of them to have different category trees

Then I expect to be able to setup MM categories like this

 ┬ Private
 └ ┬ Cat 1
   ├ ...
   └ Cat 99
 ┬ Freelancing
 └ ┬ Cat A
   ├ ...
   └ Cat Z
 - Shared

And I expect the category tree in my private budget to look like

- Cat 1
- ...
- Cat 99
- Shared

And I expect the category tree in my freelancing budget to look like

- Cat A
- ...
- Cat Z
- Shared

In order to achieve this, BB needs a "root category" setting where I would select the "Private" category group and the "Shared" category as root categories for my private budget and the "Freelancing" group + "Shared" category for the freelancing budget.