Diplomacy (#5091)

* Add diplomacy UI

* Add playercommand interface

* Add backend logic

* Implement frontend

* Send messages

* Let win conditions handle resigning properly

* Disable diplomacy in scenarios

* Add overview of pending commands

* 8 files were automatically formatted.

* make clang-tidy happy

* Make threadsafe for the testsuite

* 'src/wui/game_objectives_menu.cc' was automatically formatted.

* fix bformat

* 2 files were automatically formatted.

* fix

* Fix bformat renaming

* 2 files were automatically formatted.

* 'src/network/gamehost.cc' was automatically formatted.

* Clear clang-tidy

* Let AI make random decisions

* Forbid diplomacy in remaining scenarios

* Pull diplomacy out into separate window

* 5 files were automatically formatted.

* Delete unused variables

* Reduce AI acceptance chance

Co-authored-by: hessenfarmer <43039887+hessenfarmer@users.noreply.github.com>

* Fix Collectors teams

* Fix territorial end result

Co-authored-by: kaputtnik <6730556+frankystone@users.noreply.github.com>
Co-authored-by: hessenfarmer <43039887+hessenfarmer@users.noreply.github.com>

(by hessenfarmer)
6deb23cf9f15ed3bd7c9596cd3a42a871309b774