Game setup warning when all players are on the same team

[Langbart]

When a user is about to start a game that will make them an instant winner, a warning message should appear asking them if they really want to start the game. Below are two cases where the problem was reported. In the first case, Angen has suggested this solution in the comments.

Trac ticket #6262 by rollieoo (29/Jul/21)

After building the map the game showed with a window saying I had won and if I wanted to quit. That I and the AI player were the last ones alive in the upper left corner. This happens with every game I start now.

IRC ​0ad by Micha (1/Aug/21)

16:07 < Micha> i have one issue: when i start an offline game against the cpu, it immediately says that both players won
16:07 < Micha> and plays the victory melody
16:07 < Langbart> proably both in the same team
16:08 < Micha> THANKS!!!! :)) solved it
16:08 < Micha> obvious! didnt notice it

Layout proposal for a possible solution:

Additional remarks from elexis via IRC (1/Oct/21):

Notice that it depends on victory conditions (e.g. endless game) and allied victory / last man standing whether having everyone on the same team leads to instant victory. [...] I guess you can still capture the people who run into that issue if you would only test for conquest victory, or neglect these hypothetical non-endless victory conditions.

[Langbart]

Addition of some remarks from elexis in the description.