Reduce garbage from TextInput

[mrhelmut]

Hey there,

This is a proposal to start addressing #6360.

It removes the string manipulations and decodes each UTF8 characters individually and sends them to the event handler.
It also skips the event processing if there is no event handler attached to GameWindow.TextInput.

This should:

• remove all garbage if TextInput isn't used when pressing keys (which I believe is the most important fix here)
• greatly reduce garbage when actually using TextInput

There is still some garbage generated from this line when attaching an handler to TextInput:

MonoGame/MonoGame.Framework/SDL/SDLGameWindow.cs

Line 302 in b1a49e4

OnTextInput(this, new TextInputEventArgs(c, key));

We will have to figure out (in another PR?) what to do with event args types.

Also, it should be noted that this PR comes with a limitation (note that the original code also has this limitation): it doesn't support UTF8 characters that are encoded on more than 2 bytes (because TextInputEventArgs is limited by the char type). Chinese, Japanese, and Korean won't be supported.

This should be addressed in another PR I guess (at the same time as addressing the event args type?).

Partly fixes #6360

[harry-cpp]

Thanks @mrhelmut , merging!