Populate non-default background positions from design.ini, allow 2.8-style default positions

[in1tiate]

clang-format kinda messed up the diff, so here's what actually changed:

QStringList pos_list = {};
for (QString key : default_pos.keys()) {
  if (file_exists(ao_app->get_image_suffix(
          ao_app->get_background_path(default_pos[key]))) || // if we have 2.8-style positions, e.g. def.png, wit.webp, hld.apng
      file_exists(
          ao_app->get_image_suffix(ao_app->get_background_path(key)))) { // if we have pre-2.8-style positions, e.g. defenseempty.png
    pos_list.append(default_pos[key]);
  }
}
for (QString pos : ao_app->read_design_ini("positions", ao_app->get_background_path("design.ini")).split(",")) {
  if (file_exists(ao_app->get_image_suffix(ao_app->get_background_path(pos)))) {
    pos_list.append(pos);
  }
}

Per the title, this makes two non-breaking changes to the way pos_dropdown is populated.

• Positions specified in a background's design.ini, separated with a comma like so:

  positions=def2,pro2,custom8
  

  will be added to pos_dropdown if the corresponding files exist.
• The default positions (currently def, pro, wit, jud, hld, hlp, jur, and sea) previously checked only for AO1-style filenames (e.g. defenseempty, prohelperstand for def and hlp respectively) but will now check for 2.8-style filenames as well, allowing for the eventual removal of support for AO1-style hardcoded positions entirely down the road by providing a feasible alternative.

[oldmud0]

How do you define an overlay (desk) for these custom positions?

[in1tiate]

The overlay is supplied by an image named [position]_overlay - currently this is the only way to use overlays on custom positions.

[Crystalwarrior]

Code looks good, travis complaining about macos is annoying though