Feature: Add control over horizontal / vertical stretch for attribute form widgets

This functionality exposes two new "size" options for edit form widgets, allowing control over the horizontal and vertical stretch factors for the widget. By setting a horizontal or vertical stretch, users can control how edit widgets will relatively resize when resizing an attribute form.
For example, a user can set a higher horizontal stretch value for widgets which should "grab" more of the available horizontal space, such as for those widgets which are expected to have longer values. Similarly, the vertical stretch setting (available for select widget types) will control how widgets grow vertically when resizing forms, relative to the form dialog.
Together these options give more control to users over the exact layout and sizing of their attribute forms.
By default, the stretch values are set to "Default" which is the same as the behavior exhibited in previous versions of QGIS.

This feature was developed by Nyall Dawson