Latest upstream litehtml is often source incompatible when building qttools qlitehtml

qttools uses playground/qlitehtml as a submodule to build the qlitehtml library to be used in Assistant.

qlitehtml in turn uses a 3rd party submodule called litehtml in its sources.
This project currently does not maintain source compatibility.

This causes builds of qttools to fail when an incompatible system litehtml is picked up.

So far we always preferred a system litehtml when available, even though we clone the 3rd party sources.

To avoid further breakage, we can change by default to use the local submodule sources, rather than any found system package.

Alternatively we would have to add a configure check of some sort and only use the system package if it's compatible.