Merge pull request #22 from GuillaumeGomez/master

Add GError and FileChooserDialog
prepare rgtk to the new raw pointer reform:
- *T -> *const T = const T*
- *mut T = T*
this change all the mutable pointer *T to *mut T
*const T will come when the change is merge in Rust
fix raw pointer for last PR
Merge pull request #21 from jeremyletang/raw-pointer

prepare rgtk to the new raw pointer reform
New widget : AppChooserDialog
-> however, not all of its functionalities work since an error prevent me to test it...