Improve support for writing qdoc code in Qt Creator

It would be useful for technical writers and developers writing Qt docs to have the following features supported in the Qt Creator C++ editor:

• code completion for qdoc commands
• syntax highlighting for qdoc commands
• navigation from \l commands to link targets and macros to their definitions
• structure browser for .qdoc and .qdocinc files

We already have code completion for qdoc commands in the Qt Creator C++ code editor.