… GimpIntStore for value filling.

GimpIntComboBox was not taking ownership of the value store whereas the
newer GimpIntRadioFrame was taking ownership. As a more common practice,
I decided to leave ownership to the caller (which will therefore have
the responsibility to free the data) in the main class and property
widget APIs.

On the other hand, let's steal ownership of the store objects in the
gimp_procedure_dialog_get_int_*() functions as these are really used for
very quick and easy creation of dialogs by script writers. It would even
allow to create a GimpIntStore inline within the widget creation
function, if one wanted to.