v0.0.2 of toxext_extended_messages brought in a user configurable max
message size. This changeset implements the minimum work required for
qTox to work sanely under the new API.
* Hardcode a max message size for all friends
* If a friend negotiates a max message size below the hardcoded value
pretend they do not have the extension
* Move splitMessage out of Core to MessageProcessor
* Updates to allow for extended messages to be split
Don't add Wzero-as-null-pointer-constant by default, since on older Qt
versions that we stil support and that our CI runs agains, Qt API themselves
cause warnings which lead to build errors all over the place.
Fix#6008
