Implement a (hopefully) better way to choose the project to open.

Now allows to either:
- select a .kdev4 file and hit finish
- select a file from a project type (like CMake, QMake, Makefiles etc) and import that (providing the needed information on a second page)
- select just a directory and use the generic manager
TODO: Pre-Select the manager based on the file chosen (or for dirs thegeneric manager)