Add flex grow to IMGUIContainer

[gjh33]

unity-toolbar-extender/Editor/ToolbarCallback.cs

Line 72 in df8031d

var container = new IMGUIContainer();

The IMGUI Container doesn't have flex grow, and thus "flexible space" doesn't work. All elements will left align.

Unity 2021.1.18f1, reproduced with scene selector sample.

[gjh33]

#27 fix for issue