Previously it was only possible to have a single root-level item in a
GTreeView. This was an oversight and I didn't realize it because this
code was only ever used in the FileManager, which has one root ("/").
Also factored out item toggling into a separate function, and increase
the base indentation level so that root items can be toggled as well.
Finally, let the user toggle the selected item with the spacebar. :^)
This macro goes at the top of every CObject-derived class like so:
class SomeClass : public CObject {
C_OBJECT(SomeClass)
public:
...
At the moment, all it does is create an override for the class_name() getter
but in the future this will be used to automatically insert member functions
into these classes.
