Support dyamic resizing with `CImGui.InputText[Multiline]()`

[heyx3]

The function has an optional callback to let you resize the text buffer being edited as needed. We should take advantage of this instead of pre-allocating a buffer 2x the current size and hoping the user doesn't paste in something very large.

This will probably lead to new general-purpose ImGUI helper functions within Bplus itself. Consider contributing this helper function back to the CImGUI package.