Add Matchmaker Notification

[BlackYps]

This is a summary of the discusion in #1024

We want to have a notification in the client when someone that you could match with in the matchmaker is searching for a game.

The server should determine if someone in the player's matching range is searching and then send out a message to the client. The message should be a new message type.
It probably makes the most sense to create a new dedicated service to handle the checking and message sending. You could have a look at BroadcastService to get some inspiration.

Non-Coding tasks: Create a mockup (in photoshop or something similar) of how exactly the message would look like in the client and where it should show up.
This will guide the client implementation.

[Gatsik]

Sure! Here you are:

[BlackYps]

that's just a screenshot from the python client, isn't it?
The reason I mentioned this task is that there are some open questions that need answering. The official client has less space at the bottom. What would that look like there? If we need to increase the space there, what does it look like when it is empty? Does it even make sense to display faction buttons there, as you can multiselect factions?
There might be more questions popping up in the process of solving these.

[Gatsik]

There will always be room for bikeshedding, but this issue has existed for about six years and has been discussed many times on various platforms where player feedback is documented and can be gathered

There are also several other GitHub issues related to this in the client repository, and notification appearance should probably be discussed there, and I would argue that any such discussion should involve active players/client users -- those who will actually encounter this notification frequently

Anyway, making whatever works based on abovementioned feedback with an option to hide it would be a good first step