GtkAppChooser custom command patch

[tarnyko tarnyko net]

Hi folks, 

Could someone please review the following patch/suggestion : 

https://bugzilla.gnome.org/show_bug.cgi?id=650284#c36 

It adds a "Use custom command" on the lower part of the GtkAppChooser 
window, so users can specify an unlisted application (case of uncomplete 
mimetypes or .desktop associations). 

Screenshots are available in the comment. 

The patch itself is unfinished, I'm mostly asking for feedback about the 
general idea and design. 

Regards,
Tarnyko