Fix connect signal dialog not allowing Unicode method name

[timothyqiu]

Unicode identifiers are supported now, but currently in "Connect Signal to Method" dialog:

• Unicode characters will be removed for the default generated method name (introduced by node name or signal name).
• You'll get a "Method name must be a valid identifier" error if the method name you specified contains Unicode characters.

When generating the default method name, this PR only removes characters that does not satisfy the ID_Start / ID_Continue requirement. Possible Unicode security checks are performed when submitting the dialog.

CC @bruvzg

[YuriSizov]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.3.