More straight forward than I had expected as the major changes are all backwards compatible

Few things to do:

1. Fix up docking feature
2. The 1.89.1 tag still had IMGUI_VERSION containing 1.89.1 WIP - update in the next commit. Should probably use this instead of the exact tagged release to avoid the version string being confusing
3. Consider backwards-compatbility for the ImageButton change
4. Investigate what changes need to take advantage of the event-based input stuff (probably a separate PR)
5. Check over changelog for versions since previous 1.86 to see if there's any other significant changes for the bindings