RFC: New UI for tagging module

[zisoft]

Based on the mockup of @Ni-Ti in #15172 I have started to implement a new UI for the tagging module.

For now I have replaced the top treeview for the attached tags with a GTK Flowbox containing the tag labels:

The tag labels only contain the leave of the tags. The full hierarchy can always be shown by hovering:

The toggle switch for displaying the internal darktable tags is removed from the UI and now a module preference:

The styling is completely done in CSS, widget name is #tag-label. A tag label can have additional CSS classes based on the type/status of the tag. I have setup a basic CSS styling as follows:

Internal darktable tags are dimmed. They cannot be selected. CSS class darktable

Category tags have italic font. CSS class category

Tags marked as private get the CSS class private (no basic design from me).

If multiple images are selected and some tags are attached to only some of these images they get a dashed border. CSS class some:

/*---------------------------------------------------------
  - Tagging module                                        -
  ---------------------------------------------------------*/
flowbox.tagging
{
  padding: 0.21em;
}

.dt_flowbox_sw
{
  background-color: @field_bg;
}

#tag-label
{
  border: 1px solid @plugin_label_color;
  border-radius: 1em;
  padding: 0 0.4em;
  margin: 0.15em;
}

#tag-label:not(.darktable):selected,
#tag-label:not(.darktable).hover
{
  background-color: @selected_bg_color;
}

#tag-label.some
{
  border-style: dashed;
}

#tag-label.darktable
{
  border-color: @grey_80;
}

#tag-label.darktable label
{
  color: @grey_80;
}

#tag-label.category label
{
  font-style: italic;
}

#tag-label.private label
{
}

This part is fully functional so far and can be tested.

This is still WIP and if this doesn't get completely torn apart in the air I would continue with the dictionary part of the tagging module.

Would like to hear what you think so far.

closes #15172