Entity selection rectangle needs to take fog-of-war into account for observers

[elexis]

Since #3168 observers can change the perspective of different players, showing the fog-of-war of that specific player.

But when selecting entities with the mouse cursor, the fog-of-war is not taken into account. (Same goes for the statusbar-hotkey). This means the according needs to use g_ViewedPlayer instead of Engine.GetPlayerID() in some JS part, or GetCurrentDisplayedPlayer() in some C++ part.

[elexis]

In 18223:

Take new observermode into account in some missing places, refs #3850, #3168.

[elexis]

Mostly fixed, but two more minor things could be improved:

• Keeping tab pressed while changing the perspective keeps the status bars
• Selecting an entity then changing the perspective keeps the entity selected, even if i'ts in FoW.

[elexis]

The rest seems to be identical to #3999, GetSimState and others will probably have to be updated to use the viewed player as well.

[elexis]

In 18283:

GUIInterface / observermode cleanup as recommended by leper, fixes #3999.

Default to the viewed player in the GUIInterface to avoid passing that separately in some places, refs #3168.
Thus GetEntityState will be called for the viewed player instead of observer playerID, therefore
fixes a fog-of-war selection relict when changing the perspective, refs #3850.
Also remove some unused player arguments.

[elexis]

In 18285:

Observermode cleanup, fixes #3850.

Recalculate status bars when changing the perspective.