First phase of tool committed

Here it is... The first part of the code for my new "eraser tool" for Inkscape is in trunk now. I've been working on the use cases probably more than the code even. However this is now a starting point for people to look at and start complaining. :-) This first part only adds in the functionality to cut away parts of objects you have selected. It won't do anything if you don't have something selected, and it won't do anything if the first mode is selected (the sub-toolbar has toggles for the two modes). However it does commit all the infrastructure changes including the new icons, new drawing context, verbs, switching, etc. I had been wanting to get that in first to shake down bugs in that support work. Moving forward I'll implement the "touch-delete" mode, and support for working when no selection is made. Probably a few more enhancements will come in as people try it out and give some feedback. I'm most excited about the pending "touch-delete", since it has a feel that strikes me as very "vector" and can differentiate the usage from raster drawing packages.