Improved ally behavior

[tlsa]

Preliminary checks

• [X] I've checked that there aren't other open issues on the same topic.

Describe the problem requiring a solution

When playing maps where two or more players are allied, there are various regressions compared to the behavior of the original game that are particularly apparent in multi-player:

• Sharing mage guild spells It used to be possible for an allied player to remove the creatures from a castle and allow their friend to take the castle to learn a good spell. The player could then re-take the castle on the next turn.

  This behavior is possible at the moment in fheroes2 only when the players are not allied, because allies are prevented from visiting allies castles.

• Battling with allied heroes At the moment with fheroes2, allies can't battle each others heroes. It used to be fun in the original game, when playing with a friend, at the end of the game to load the last autosave and let both players best heroes fight.

Describe the possible solution

A good solution for this would be an improved ally mechanic.

• [ ] Allow players to visit their allies castles, mines, etc without taking them
  • This will allow players to learn the spells at an allied mage guild without the clunky take, re-take mechanic of the original game.
  • If an allied castle contains creatures, the castle is visited without causing a battle.
  • If there is a hero in a castle, it is not possible for an ally to visit without breaking the alliance and fighting the resident hero.
• [ ] Add feature to break an alliance
  • There would be a new menu showing alliances and allowing the player to break them.
  • After an alliance is broken, the former allies can take each others mines/castles/etc and fight each others heroes.

Additional info

No response

[ihhub]

This issue correlates with #1321