Add char input

[Jerald]

Based on the comments and information from #782, this is an alternative approach to providing a form of simple text input in bevy.

This adds the CharInput event to bevy_input and modifies bevy_winit to dispatch the event when the winit ReceivedCharacter event fires.

Only one of either this PR or #805 should realistically be merged.

[Jerald]

I've now added #805 which is a more window-focused alternative to this approach. I'm leaving both open so the preferred one can be chosen, closing the other one.

[cart]

closed in favor of #805

[stanle-cs]

@cart I don't know if it's appropriate to ask here, but ran into this issue today. Is there a reason why #805 is favored over #804? I tried to use <ReceivedCharacter> and realized that it is part of the window plugin, so I can't use it with the minimal plugin group. Tried to find something similar to this at a lower level but doesn't look like we have an easy way to get the char from either ScanCode or KeyCode unless I'm mistaken.

[cart]

@stanle-cs Character input pretty much always happens within the context of a window (and on some platforms, must happen in the context of a window). We might later add "windowless input" support for platforms that support it, but that is a corner case to a degree that I'm not sure we should be focusing on right now.