More intro screen features

https://github.com/kronosaur/Transcendence/pull/67
https://github.com/kronosaur/Mammoth/pull/29

Current status

• No active bugs or errors
• No pending commits

New Events

• OnGlobalIntroStarted: Fires on all types when the intro system is created. If the POV is changed as a result of firing the event, then the intro screen skips auto-creating ships
• This allows mods to introduce customized intro screens
• OnGlobalIntroCommand: Fires on all types when the player inputs a string beginning with ~ in the ! input box.
• aCommand: The command that was entered, excluding the `~` from the original input.
• The goal of this feature is to generalize the ! ship class input box into a quick function input menu.

New Intro Keys

NOTE: Uppercase and lowercase keys may have distinct bindings.

• I and i now bring up help text for intro screen key bindings.
• EDIT: This binding was changed from h to I and i
• K now destroys ALL ships
• N now cycles to the previous ship class
• S now toggles combat sounds (silent by default)