schematic-view: Properly name GtkBuilder variables as `builder`.

No `gui` is not correct, the builder just parses an UI XML file but is not tied to the lifetime of the UI itself.
SchematicView, TextBoxItem: Further `gui` by `builder` replacing.
SchamticView,LogView: Embed LogView into a scrolled window, make the main.ui paned.
NetListEditor,NetListHelper,NgSpice: Continue replacment of error dialogs by LogStore.