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 possible to do several actions (eg delete)
on the current selected pictures. The selection can be different from
the picture displayed on screen. To avoid confusing the user, in this
mode the displayed picture becomes automatically the currently selected
picture.

There is also the same problem with full_preview mode, this commit also
does the same thing in full_preview mode.

So when unzooming, the previous selection is lost.

TODO:
- This commit removes the ability to circle between multiselected
pictures as it redefines the current selection. Perhaps the good way is
to full preview the selected picture instead of the picture under the
mouse...
- In my opinion people that use full_preview mode doesn't want their
previous selection to be lost. Perhaps another solution would be to
remove the ability to delete/go previous/go next picture in this mode.
So the previous selection can still be kept even after pressing the 'z'
key, but the user cannot delete a picture he doesn't want by accident.