This change allows the attributes of multiple features to be edited
simultaneously. It is enabled when the attribute table dialog is in
"form mode", via a new "multi edit" button on the toolbar.

In this mode, attribute value changes will apply to all selected
features. New widgets appear next to each editor widget allowing for
display of the current multi-edit state and for rolling back changes
on a field-by-field basis.

Changes are made as a single edit command, so pressing undo will
rollback the attribute changes for all selected features at once.

Multiedit mode is only available for auto generated and drag and
drop forms - it is not supported by custom ui forms.

Sponsored by Kanton Basel Stadt