[plt-scheme] 202.3

The exp-tagged code in CVS for MrEd and MzScheme is now version 202.3.

MrEd 202.3 introduces the tab-panel% class, which implements a panel
with a row of selectable tabs at the top. When the user selects a tab,
a callback function is invoked. The tab-panel% class is a subclass of
vertical-panel%.

The new class provides only tab-control primitives. It does not do
anything with a tab selection, such as changing the set of controls
displayed in the panel. I imagine that a framework/mrlib class will
appear soon to implement content-swapping based on the tab selection.

The class still needs more work. Under OS X, the panel content is not
drawn in a raised boxed, which means that a tab-panel% looks odd when
it doesn't occupy a whole frame. Under X, a disabled panel's tabs look
enabled. Under Windows and X, there's currently no way to change tabs
via the keyboard. These problems will be resolved in the coming weeks.

The documentation that was built last night (and is thus available for
updating in Help Desk) gets the order wrong for tab-panel%'s
initialization argument. The correct starting order is choices-list,
parent, then callback. Corrected documentation will become available
tonight.

There are no MzScheme changes in 202.3.

Matthew