Update Dear Imgui to 1.79 with Docking Feature

[AustinEast]

This PR updates Dear Imgui to the Docking branch, based on 1.79 (the latest release). It also updates the HL bindings and Haxe API, with only minor breaking changes (ill list them below). I definitely understand if you aren't interested in merging in a feature that's technically still in testing - but it is very useful, it is togglable (it's turned off by default), and I can say from experience that it's fairly stable.

One thing to note - if accepted, this PR should be merged in after PR #1, since this branch was branched off of that (ive been using both features in my own codebase). Sorry about the inconvenience!

Changes:

• Updates extensions/imgui to use the latest docking branch that has just been merged with 1.79 (specifically this commit)
• Stops using RenderDrawListsFn in HL bindings (as it's been made obsolete in newer imgui versions), and updates the Render method to compensate
• Adds dockSpace() method to Haxe API
• Adds setConfigFlags() method to Haxe API. This is useful in general, but is also required to turn on the docking feature (like so: ImGui.setConfigFlags(ImGuiConfigFlags.DockingEnable);)
• Breaking - changes ImGuiStyle.TabMinWidthForUnselectedCloseButton into ImGuiStyle.TabMinWidthForCloseButton
• Breaking - changes ImGui. openPopupOnItemClick() return type from a Bool into a Void

[haddock7]

Thanks again, I wish I had more high-quality pull requests like this one! I could definitely show it as a good example at my work :)
I have no problem merging the docking feature, as it has been around for 2 years now, so it should be considered stable.

Please make the tiny change in the first PR, and I'll merge them.