Added the Item Editor GUI.
Started to make the Item Editor functional: the Add button works, as does item selection.
Added item deletion to the Item Editor.
Added live-update of the item name to the Item Editor.
Fixed a bug where typing in the built-in search bar in the Item Editor item list would trigger a ::changed on the Name entry box, which would kill the program because the list item is temporarily deselected.
Added price field validation to the Item Editor. Thanks to Company from irc.gimp.net/#gtk+ for help with this.