An EBMessage associated with an EditPane that is sent just before its caret
position changes in a "major way" to another location in the same Buffer.
These messages are tracked by the Navigator plugin,
and other interested plugins.
jEdit plugins such as SideKick, Tags, Jump, CscopeFinder, etc, should
emit this message whenever the user wants to jump from one position
to another in the same buffer.
For jumps to a different buffer entirely, it is not necessary for plugins
to send any message, since BufferChanging is sent by jEdit whenever
EditPane.setBuffer() is called, and it serves as a PositionChanging message