Fix HTML errors in filter_form_api.php

Fix HTML issues reported by Nu Html Checker:

• Error: Row 7 of a row group established by a tbody element has no
  cells beginning on it
• Warning: A table row was 0 columns wide, which is less than the column
  count established by the first row (8) in filter_form_draw_inputs()

Add a new method TableGridLayout::render_spacer() to print a HTML code
for a spacer row of current table.