[qmltc] name clashes with generated class names

`qt_add_qml_module` does not set the a namespace by default. however this easily leads to clashes with application code (e.g. an innocent looking `Display.qml` fill fail to compile due to `typedef struct _XDisplay Display`).

this makes me wonder if it would be more robust to provide a reasonable default for TYPE_COMPILER_NAMESPACE (e.g. "qmltc"), as it provides an initial safety net to protect against such name clashes (principle of least surprise).
not sure if there are any downsides of this approach (maybe debug info get a little larger due to larger symbol names). thoughts?