* Items are now objects that can be retrieved and manipulated.
* clip rect is used for left and right panes
* initial support for items to do their own custom drawing by also
doing a clip rect for them
* a custom background color can be specified for the item.
* a command or function can be executed on item click.
* Introduced functionality to easily hide or show all or specific items.
* Better handling of deprecated get_items()
* Spacing is automatically added to items and cleaned on deprecated
items.
* Default items where separated and given the names:
doc:file, doc:position, doc:indentation, doc:lines,
doc:line-ending, core.commandview.
* Some default right or left click actions where given to the default
items.
* Started adding required bits to support dragging to left and right
panes when some items aren't visible.
Note: should work well enough already but maybe some repetitive stuff can be
cleaned out.
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