Eliminate usage of opening date for categories

Set the opening date for newly created categories to 1900-01-01 and fix
existing ones to have that date during the consistency check. Since the
opening date of an income/expense account (aka category) is not used
throughout the application, it should not interfer with the check that
the opening dates of all accounts referenced in a transaction are prior
to the transaction's post date. Setting the opening date of a category
to the above mentioned value makes sure that a category is always
'opened' before the transaction took place.

The UI elements for the opening date have been removed from the
account/category edit dialog as they are not used anymore.

BUG: 313793
GUI: