Various fixes

* Rename B_USER_* to B_SYSTEM_* for Haiku PM
* Add the template files to the project when creating one
* Make sure the resulting executable is inside the project directory
  when building
* Resize the error window
* Remove DEL shortcut for removing files from a project because it
  didn't work and wasn't recognized correctly by Haiku (i.e. no icon)
Merge pull request #9 from tgkokk/master

Various fixes