Investigation into #126306 showed that the texture tab shown would actually be
for the active image paint brush, but people used it to set up a texture for
the fluid modifier settings anyway. Now properly register the texture user for
the UI, so the texture properties tab will always be displayed when there is a
fluid modifier with a "Flow" fluid type.
Main issue is that fluid modifiers weren't handled by the
`BKE_modifiers_foreach_tex_link()` iterator.
While this could be handled as another special case in
`buttons_texture_modifier_foreach()`, I updated the texture-link iterators to
support texture properties that are not directly stored in the modifier, but in
some nested data. Seems like this should be supported generally. It's enabled
here by passing a pointer-property pair, rather than just a property name.
Pull Request: https://projects.blender.org/blender/blender/pulls/126893
48208ab2d8