Goober5000

Add member types, tweak function signatures, and add a couple functions

These were coded for the `split_str` refactor, but are generally useful elsewhere as well. 1. Use SCP_string in some utility functions where appropriate 2. Avoid copying C-strings in some utility...

A consistent friendly way to get a waypoint name. Follow-up to #7103. ~~In draft until #7103 is merged.~~

When taking a screenshot, the ImgUi widgets are not written to the image. This might be fine for the ship lab, but is not so fine for the in-game options...

Improve the speed of the SEXP filter by not redrawing the combo box until the box is fully populated.

These functions accept a size parameter that *includes* the null terminator, so tweak function calls accordingly.