Added ComboBox, HorizontalLayout, TreeNode, TreeModel, TreeView. Widget is now descendant of EventSource. Improved color management (color prefixes).
Event handling
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Keyboard event propagation
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top widget -> focuswidget -> focuswidget's parent -> focuswidget's parent's parent -> ...and so forth
1 top window
4 window
3 container
2 edit box
Other event propagation
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