Tree improvements

merges in changes made in my fork of Ken Scambler's ScalaSwingTreeWrapper.

- adds missing methods to tree, such as visibleRowCount, makeVisible
- corrects some scala style issues (getter methods should not have empty parens)
- some small improvements (case classes are final, unnecessary abstract vals replaced by defs)
- renaming in tree selection: empty -> isEmpty; count -> size (which is more coherent with existing scala collections terminology)
- renamed Renderer.labelled (british english) to .labeled (american english)
- add .idea paths to .gitignore
- bump version to 1.5-SNAPSHOT. update gpg plugin. some small build.sbt improvements