better mouse mode icon location specification

[Eric Pettersen]

The current scheme for mouse mode icons isn't very extensible since the icon has to go into the mouse_mode tool itself. Need to be able to specify an icon path, not just name.

[Tom Goddard]

Currently the icons get into the mouse modes documentation by being copied from the mouse modes bundle. If the icons are in different locations we will need a different solution for getting them into the documentation. One idea is that any icon in the documentation will need to have a copy of the icon file in the documentation.

[Tom Goddard]

Made each bundle that defines a mouse mode keep the icon in its own bundle. Registering the mode gives the full path to the icon. For documentation, a Makefile under docs knows where all the mouse mode icons are so they can be copied into the documentation.