This commit adds a new mode to the attribute table dialog for searching
and filtering features. When activated (using a button on the toolbar
or by pressng CTRL+F), the dialog will switch to form view and all
widgets are replaced with their search widget wrapper variant.
Alongside each widget is a tool button with options for controlling
the search/filter behaviour for that field, eg "equal to", "not equal
to", "is null", "greater than", etc.., with the options presented
matching themselves to the corresponding field and widget type.
New buttons appear at the bottom of the form for either selecting
matching features (with options for add to selection/remove from
selection/select within current selection) or filtering features
in the table (with options for adding features to a current filter
or further restricting a current filter).
Sponsored by SIGE
