Combocell popup closes immediately the first time it is used

[Scott Oonk]

Change gnc_item_list to emit a "changed_item" signal instead of an
"activate_item" signal when the tree-view emits a "changed" signal. 
Emit the "activate_item" signal when an item is selected with the
mouse or by hitting Return on the keyboard.

This fixes the problem where the combocell popup closes immediately
the first time it is used.  It also leaves the popup open when moving
through the list with the up and down arrow keys.

Scott