Fog/Frost and Securing: added plane cover implementation (#450)

* Fog/Frost and Securing: added plane cover implementation

Starting cold&dark in winter with no plane cover makes the plane frosted.
Use the "clean aircraft" button to remove frost and snow.
To prevent, a plane cover ca be applied using the aircraft-securing dialog.

Fog/Frost refactored regarding cleaning/inducing:
- Added /fdm/jsbsim/heat/init-moisture to quick-add fog/frost condition
- Aircraft cleaning button now also removes frost/fog (it's being scraped/wiped away)

* Cover 3D-model from @TheFGFSEagle, thank you very much :)

Documentation.md

@@ -24,6 +24,9 @@ messages will give a hint on the condition and also the trend (give it some time
 Opening the windows may also be an option to cool down quickly.  
 If the text messages annoy you, you can disable them by unchecking the `Show hint messages` checkbox.
 
+When starting cold&dark in freezing temperatures, the plane may freeze over. In this case you need to clean it (see aircraft options dialog).
+To prevent this, you should apply the plane cover when securing your craft.
+
 
 ### Complex engine procedures
 Unchecking `Complex engine procedures and failures` allows a fairly basic engine mode where no priming is needed (just activate
@@ -131,6 +134,7 @@ Additionally to the default ones, the c182 knows the following:
 - `/engines/engine/manual-roughness-factor` (`0.0` to `1.0`) to induce engine roughness
 - `/engines/engine/manual-power-reduction-pct` (`0.0` to `1.0`) to make the engine less efficient
 - `/engines/engine/kill-engine` immediately kills the engine if set to `1`.
+- `/fdm/jsbsim/heat/init-moisture` (`0.0` to `1.0`) quick-induce fog/frost. The setting will be hold indefinitely, so reset to `0`.
 
 ### KAP 140 Autopilot
 The C182S features a highly detailed and realistic Bendix/Kind KAP 140 Autopilot. You can operate it like described in the manufacturers POH.