Bump required cmake version to 2.8.4 to make it more Windows compatible.
Fix add and remove button behavior (project browser).

 * Improve iterator handling.
 * Problems:
    - Removal of files does not work (project browser vs. project objects not
      synchronised).
    - How to differentiate between project and buildsystem files?
      In future implementations the buildsystem is a plugin so project browser
      has only to care about source files. Therefore hardcoded workaround is
      ok to fix this until buildsystem plugin comes.
 * There are some duplicate code parts (TreePath and TreeIter stuff).
Merge branch 'project-browser-iters'