This is done by adding a new button type, GRIP, similar to other numbuttons
(scroll, slider, ...), which here controls the preview height.
Then, we add a new DNA struct to be able to save that height in Blend files
(note I choose not to use Panel struct for this, because we would then have the
same limitation we used to have with uiLists, only one preview per panel
and no preview outside panel).
This implies a change to template_preview UI RNA/py API (each preview needs an ID),
but this is backward compatible, as by default datablock type will be used if no ID is
given (which means e.g. all material previews with no ID will have same height).
Reviewers: brecht
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D342
4faef1e10c