Category windows: Layout tweaks

* Category window:
  + Simplify layout using Group layouts.
  + Center buttons
  + Only enable "Edit" and "Remove" buttons if not a superitem
    (Income/Spending) is selected. Those can't be renamed or
    deleted.

* Edit category window
  + Layout tweaks

* Add category window:
  + Use radio buttons for Income/Spending instead of one checkbox.
  + After adding a category, close the window.
    When you're entering many new categories it may be nice to keep
    the window open. But if you don't happen to that new category
    added to the list in the main category window, you're never sure
    it got added alright.
  + Disable OK button when window opens and the name field is empty.
  + Layout tweaks

* Remove category window:
  + Only enable "OK" button if not a superitem (Income/Spending) is
    selected. Those can't be chosen as subsitute for the transaction
    of the to be removed category.
  + Layout tweaks