Summary:
For that we need a QTextStream backed by a QIODevice. We have two
implementation, one backed by a QBuffer and one by a QFile, rplacing
most of the various sub-classes we had previously.

This means that the internal buffer is now a QByteArray rather than a
QString. That however turns out to not be a problem at all, as
QWebEnginePage::setHtml() basically just calls setContent(html.toUtf8()),
ie. we need a QByteArray in the end anyway.

Another side-effect of the QIODevice usage is that this now becomes a lot
more sensitive to being in the correct state, therefore also the test code
needs to properly call begin/end now.

In order to employ this to the full extend the next step is passing this
along to all render methods and writing into it directly rather than using
intermediate buffers, as well as adding a custom Grantlee item for nested
templates so we can do this with streaming too.

Reviewers: knauss

Reviewed By: knauss

Subscribers: mlaurent, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8087