*imgui*
fixed:
- Fixed bug where the scaling button in a window could get unreachable due to lying above another child window. This is solved by letting the 'ButtonBehaviour' function accept an argument called 'force_focus', which if provided with TRUE will ignore if the window is active.
- Made reseting the ImGui environment explicit, so it is no longer a part of the Render function. This is mainly because the Nebula game loop has this particular order:
    Handle input events.
    Render (including ImGui).
    Run user code (create new ImGui stuff).

Which basically causes the character input and other inputs to be 0 at the time where the UI is created. We can instead reset the environment prior to handling input events, which makes it integrate neatly with Nebula.
*imgui*
- Added flag which can be used on windows in order for them to not have a resize button, but still act like a tooltip in the sense that they always rescale to fit the size of the content.