Recommend explicit translation context

The documentation and examples so far recommend using an automatic translation context for non-id-based translations:

• For C++, this is typically the name of the QObject subclass
• For .ui files, it is the class name
• For QML, it is the file name

Anyhow, this automatic context is often a poor approximation of the semantic context:

• Class and form names are often very technical, not actually giving translators much hints
• Parsing e.g. C++ for deriving the correct context at runtime is complex and error prone
• They are often too fine-grained, leading to lots of contexts with little text, hammering translation reuse
• Just renaming classes and files will lead to outdated translations

We should steer users away from relying too much on the automatic context:

• For QML, you can use pragma Translator on a file level
• For C++, e.g. Qt Creator adapted a pattern to define an explicit context on plugin level