For force generate ID property for event handler.

Completed:
1. Event handler dialog construct
2. JS code exporting.
3. JS Preview.

Introduced ControlGroup as base widget for ButtonGroup, RadioGroup
and CheckboxGroup, then base horizontal and vertical ButtonGroup
on ButtonGroup

…l in Footer

For hover effect.

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We can define a "morph" function for a zone so that When adding
a child, it can be morphed to another type. With this feature, we
can have a generic widget in the palette which can adapt
to different types of containers when dropped on them.

Looking for parent zone with the same name. If found, extend it
with descendant zone, otherwize add a new zone.

Sorting is still OK without this change and it causes bad
appearance of ButtonGroup and ThumbnailSplitList

Previous algorithm only supports one zone containers or Headers,
and it works hard to determine the position for insertion. The new
algorithm firstly tries to insert to the zone of the nearest
sibling using enhenced insertChildAfter/Before, if fails, uses
addChild to automatically find a zone to insert.

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Conflicts:
	src/js/widgets.js

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Conflicts:
	src/js/views/property.js

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