Rename AddFilterDialog to ModifyFilterDialog

since we will simply be using the same dialog to modify existing
filters.
sql/accounts/Create.2.sql: Make the id AUTOINCREMENT
Account: Reallt load filters in load_filters
Filter: Also save the filter's ID

We need it to distinguish and update different Filters, their 'content'
field won't be sufficient.
modify-filter-dialog: Also fix the class name

It's ModifyFilterDialog now, not AddFilterDialog
FilterPage: Get the filters from the Account
FilterListEntry: Operate on Filter instances
modify-filter-dialog: Make another string translatable