Crashed for example in the case of template instantiation. To make sure
the Function* stays valid, we need to not only keep around the Snapshot
but the whole LookupContext.

Done with Roberto Raggi.

Fixes: Do not activate member completion when class names are used as base of a DOT or ARROW expression.

For example,

struct {
  int a, b
} foo;

int main() {
  foo.  <-
}

Reviewed-by: Roberto Raggi

Spaces after the opening brace would cause SIGNAL/SLOT completion to be
disabled along with function completion. Now function completion is
checked at a later stage.

It didn't make sure its Function* stayed valid. Now it does, by keeping
a copy of the list of CppDocuments (the snapshot) around.

Reviewed-by: Roberto Raggi

Some people don't like this.

It is now possible to choose between case-sensitive or case-insensitive
completion. Also, you can now disable the automatic insertion of braces.

This change removes a number of wrong usages of the CppModelManager::documents()/document().

The code completion was keeping around references to old documents after
the completion finished. This caused documents to stay in memory when
unloading projects, up until the next time you used the completion.