Add slot-related methods to SubPackageMgr.
Fix a bug where a slot's default value was always being returned as a
String.
Add undo/redo support for changing slot values.
Allow querying of a MultiUndoOp's size.
Explicitly setting a slot's value to the slot's default value should be
permitted (not ignored).
New method for determining a font's height (in pixels).
Initial commit of the SlotValuePropertyPage, with support for Text,
Integer and Boolean types (File/Directory still to be implemented).
Add parameter slots into the tool-tip message for sub-package
pictograms.
Ensure that slot changes (PackageMgr and SubPackageMgr) cause the
correct notifications to be sent to listeners.