Update FLTK to 1.4.5

[dannye]

1. Update FLTK to 1.4.5
2. Use out-of-tree cmake build for FLTK (will be required starting in FLTK 1.5, planned to be released later this year)
   • This refers to cmake -B build, cmake --build build, and cmake --install build
3. Set FLTK_ABI_VERSION to 10405 (doesn't enable anything really essential, but nice to have)
4. Update HTML_View scrollbar slider box type (possible as of FLTK 1.4.4)