QDoc: Remove `QmlCodeParser::m_engine`

As part of producing a documentation set, QDoc parses a series of
user-defined input source files in various languages to extract the
user-provided documentation and certain source-code level elements.

For QML source files, the `QmlCodeParser` class is used, taking care of
reading the QML code and build an internal representation for some of
its elements.

`QmlCodeParser` uses the `QQmlJs` parsing api to do so.
As part of using the `QQmlJs` parsing api, `QmlCodeParser` has to build
a `QQmlJs::Engine`, `QQmlJs::Lexer` and `QQmlJsEngine::Parser`.

Currently, `QmlCodeParser` stores the engine element as
an instance member, `m_engine`.
Supposedly, this is intended as a way to avoid building the element more
than once.

Building a `QQmlJs::Engine` more than once does not provide any
meaningful overhead, moreover not in the code path where it is used.
Furthermore, `m_engine` is used only a few times, in
`QmlCodeParser::parseSourceFile`, the entry point for parsing a QML
source file.

Hence, to reduce the scope of `m_engine` and avoid requiring an
out-of-scope initialization for it, `m_engine` is now removed in favor
of a local instance in `QmlCodeParser::parseSourceFile`.

Change-Id: I73f23ec06c1fa3ba3fe50ff4dffc9ff9229c114d
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

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