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 :)
This is a duplicate of the upper part from 401 (maybe split it).