Remove use of obsolete ImGui functions

[Legulysse]

This PR contains migration commits to allow imgui-sfml to be compiled with the flag IMGUI_DISABLE_OBSOLETE_FUNCTIONS.
I put them in the order of imgui updates for easier integration if needed.

Those commits will force the project to raise the minimum imgui version supported, so it may not be welcome right now.
However, it can be useful for people willing to enforce this compilation flag on their projects.
I am not familiar with cmake and thus I am unable to tweak/test the version settings on this part.

Please feel free to use those commits as you see fit, either by cherry-picking them at a later time, or by merging them into a different branch.

This PR is tied to this ticket : #301

[ChrisThrasher]

Now that CI is checking for the use of obsolete functions, if we want to remove more obsolete functions then what we're really doing is raising the minimum ImGui requirement. That's a valid thing to consider, I just want to make sure we're on the same page about what it entails.

For now I'm going to close this PR because as it stands I'm not yet sure I want to raise the ImGui requirement for ImGui-SFML 2. ImGui-SFML 3 on the other hand certainly ought to raise that requirement. I'm not sure what version to pick but I think something newer is in order.