app: Implement zooming in GimpContainerTreeView via mouse wheel and touchpad gestures

This MR implements zooming in GimpContainerTreeView via mouse wheel and touchpad gestures. This means that sizes of previews in docked tabs such as Palettes, Layers, Gradients and so on can now be resized without going into menu. This makes inspection of the previews easier. For instance, one use case which I found very convenient is quickly zooming in to inspect some gradient that I maybe want to select and then zooming out to get an overview of gradients again.

gimp_view_size_get_larger and gimp_view_size_get_smaller should probably live in core-enums.h/c, but because the core-enums.c file is generated I decided to place the functions locally for now. It would be great to have a better solution, maybe there's a file for such utilities already?