Move slider/gallery submodules to separate projects?

Problem/Motivation

I think it would make sense to move slider/gallery submodules (bs_slider_bootstrap, bs_slider_tiny_slider and bs_slider_swiper to separate projects. This would make managing this submodules easier. Check with @berdir before doing this, but this would make sense to be done before going from sandbox to public project.

In future we should add some very minimal demo like slider implementation in core bs_slider module that would be used more for testing then real usage.