expander-row: Retract on click

Currently, HdyExpanderRow is a bit counter intuitive to use as clicking the row expands it, but clicking it again doesn't retract it: you have to use the now revealed toggle button. If you nest a listbox in it, it will be surrounded by a 2px padding and hovering the nested list will show that the enclosing row is hovered too.

All of this makes the expander row look and behave terribly bad.

I suggest to:

• Drop the toggle button.
• Retract the expander on second clicks.

I suggested to acompany it with a widget handling nested rows, it would have:

• No row padding.
• A reveal property.
• A revealer you could bind to your expander row.
• A listbox in that revealer.
• Maybe a getter for that listbox.
• Maybe a header where you could put a separator that would hence disappear when the row is not revealed.

This would break both the behavior of HdyExpanderRow and hence should be introduced with a hard library version change like libhandy-0.1.