Fixed some range_slider issues

[klytje]

I fixed the range_slider issues we discussed on Discord. More specifically:

• Scrolling now checks for collisions.
• I added an overload to range_slider which takes an additional float offset_from_center. This is intended to be in the range [-0.5, 0.5] and specifies where on the shape the ´value´ will be. Setting this to 0.5 allows total overlap of the thumbs, 0 allows exactly no overlap, and -0.5 blocks movement when the spacing between them is one thumb width.

Please check the new basic_range_slider_offset_base and how basic_range_slider interacts with it; I'm not entirely sure what I've done there is legal.

Name improvements for offset_from_center are happily accepted :)

Note: this should probably only be accepted after my other PR since this one is based on that.

[djowel]

Awesome! This is looking great, @klytje!

BTW, I've been wanting to ask: can you please do a squash commit to minimize the history?

[djowel]

Ooops. Conflicts:

[klytje]

Should be fixed now :)