Consider a better way to distinguish between GTK+ and relm widgets in the view! macro

[antoyo]

Currently, a widget with a module separator like gtk::Button is considered a gtk widget while one without is considered a relm widget.
This is confusing.
I cannot check for the gtk prefix because other crates can provide gtk widgets like webkit2gtk.

An idea is to require to prepend a symbol in front a relm widgets like @Button.
I don't like this idea since it makes a distinction between gtk and relm widgets.

[antoyo]

Trying to create traits that will be used for both gtk and relm widgets in order to keep the same syntax.

[antoyo]

Fixed by #149.
Finally, I decided to allow the use of the $ symbol to be able to use a path for the name of a relm widget.
The old syntax (identifier) still works for relm widget to maintain backward compatibility.