Observer Mode - "Enter" causes turns to advance twice

[QuintillusCFC]

Documenting this since it will be forgotten eventually otherwise...

In Observer Mode, frequently, if you press "Enter", it advances two turns. This appears to be due to the end turn button getting invoked as well as the enter key.

Pressing the end turn button with the mouse only advances one turn.

Interestingly, pressing the NumPad Enter key only advances one turn.

If you fortify all units before entering observer mode on turn 0, regular-keyboard enter key (usually?) only advances one turn.

Not a major issue at this point since Observer Mode is basically a dev feature at this point, but we should fix it eventually as it'll also be a useful mode for mod creators.