Issue #9527: Update canvas image on single click in Image Dockable

This patch attempts to resolve #9527 (closed).

As noted in the report, a single click on an image in Image Dockable will update the layers and Navigation views. The canvas will not update unless the item is double-clicked however. This patch adds a "select_item" function to GimpImageView so that the canvas will update on a single click as well.

@pixelmixer, Hi! If you have time, I'd appreciate any testing or feedback you could provide for this patch. Hopefully it helps resolve the issue.