views/lighttable: Select the current displayed picture when zoom == 1.

In filemanager mode and zoomable table with zoom == 1 (only one picture
displayed on screen), it is still possible to do several actions using
selected pictures tool (eg delete) on the current selected pictures.

The selection can be different from the picture displayed on screen.
To avoid confusing the user, when zoom == 1 the displayed picture becomes
automatically the currently selected picture so that the selected
pictures tool acts on the current displayed picture.

When unzooming, the previous selection is lost.
selection: Automatically select the full previewed picture.

This feature need to be discussed with darktable community as I don't
see a need to navigate through picture in full preview mode. Moreover I
think this is a bad idea to modify current selection when going to full
preview mode.

A better solution would be to disable any action on the full previewed
picture (including navigation with left/right/scroll).