- remove image from group before removing/deleting it. [done]
    - add duplicates to the same group as the original image. [done]

'G' off:
========

    - display all images. [done]
    - draw a border around all the images in the group when hovering over an image with the mouse. [done]
    - merge border of adjacent images. [done]
    - add config option whether border is to be painted or not. [done]
    - disable grouping accels (ctrl-g, ctrl-shift-g) so that people don't hide their images where they won't find them again without noticing. [done]

'G' on:
=======

    - only display a single image ("representative") from each group. [done]
    - show the 'G' in a corner of the thumbnail. [done]
    - expand the group temporarily when clicking the 'G'. [done]
    - when expanding a group while another group is already expanded, then also collapse the latter. [done]
    - show the 'G' for all images in a temporarily expanded group. [done]
    - draw the border around the images of the temporarily expanded group as if grouping was turned off, just in a different color. [done]
    - highlight the 'G' for the representative ... [done]
    - ... and when clicking the 'G' of another image in the expanded group, then this image shall become the representative. [done]
    - collapse the group when clicking the 'G' of the representative. [done]
- ctrl/shift clicking the 'G' of a group selects all the images in the group. [done for expanded groups]
    - when a group is expanded and another image is selected, then this image can be joined to the group by ctrl-g. [done]
    - when multiple images are selected and no group is expanded, then ctrl-g will merge them into a new group. [done]
    - remove an image with ctrl-shift-g from a group. [done]
    - an image is the representative of the group iff id == group_id. [done]
    - expand the group after creating a duplicate and adding it. [done]