Meta Editor super small fixes

- new editor loading on linux fixed (incorrect qreal_editor_path generation in .pro)
- generated files deletion after fail of load/unload plugin removed
- properties "name of the directory" and "path to qreal sources" now can be specified by standart choose-directory dialog (property type "DirectoryPath")
- error messaging about building/loading/unloading new plugin improved
- [general feature] default item name now without brackets