Rearrange filter dialog widget

There were some issues withthe filter dialog:

• The form tags are not placed correctly in the layout, crossing inner
  div boundaries.
• The section at the bottom is not hidden when the widget is collapsed.
• Only one form is used, but there are several actions that don't need
  to share all form fields.

In order to improve some of the dialog options, the layour has been
rearranged:

• Move the buttons toolbar to the top of the widget, following the
  general style of other widgets.
• Move the "text search" field inside the filter parameters area. This
  is part of the filter properties and should be managed and rendered by
  the dedicated code.
• Assign each area its own form. Time ago, the filter submit would parse
  all the fields as a whole, but now there's the option to evaluate only
  the submitted fields, incrementally over the affected filter. This
  allows to separate the header search field, filter load dorwpdowns, etc,
  into individual forms.

Fixes: 0025109