Fix selection when one image is in the gallery we rename it and add buttons for scrolling up and down.

When there are only one image in the gallery and we rename it, the image disappears.

The commit 6e2f1276 handle this case by adding the event G_FILE_MONITOR_EVENT_RENAMED in the monitors.

Without this event, the monitor delete it first, which that make the selection disappears, and after create it but without selecting it.

Also, at the call of the insertion the row is still empty so we cannot select the image.

Concerning the commit 6a4c67e9, I saw when we change the position of the gallery to left or right, there are no buttons anymore.

This feature add this buttons. It seems GTK_TEXT_DIR_LTR and GTK_TEXT_DIR_RTL works well too for scrolling up and down.

Please see the commit for more details.