Add more resources to the D2k mod

1. With the map importer for Dune 2000 maps in the making (#7135), we will need a few more things:
**a) add "missing" tiles to the `arrakis.yaml` tileset file**
    While we (*may, but questionably*) have all different tiles referenced in the file and this is good enough for the map editor, the map import does not have everything it needs (terrain-wise), which results in incorrect tiles (nothing deal-breaking, but they have to be then manually replaced via the map editor). I've gone over this in several other tickets and on IRC. (#5875)
**b) add House Corrino, Fremen, Smugglers and Mercenaries as non-playable subfactions**
    This is required for importing missions.

2. Also I am adding a few voice notifications that we are not, but should be using.
a) The code that will use the `EnemyUnitsApproaching` notification is currently being worked on.
b) `Guarding` should really be used by the existing Guard command.
c) Once the new UI is in place and the upgrades for the production structures are implemented, the `Upgrading` notification should be used. (#6043, #3211)

3. Please don't bash me for the empty `corrino.yaml`. I will need some advice on what structures does this faction have that are visually different from the other houses to populate it properly.

4. Added a few more loading screen messages :)
It is a duplicate. And we do need only one for the map editor. But when importing maps, we need the correct tile with the correct ID, otherwise we get random tiles. We can live with that, but it means that the importnwill be only partial and then the person has to fix parts of the map in the editor.