Performance: QDeclarativeText does unnecessary layout steps for RichText

When creating a QDeclarativeText component with Text.RichText format, initializations cause unnecessary layouts to be carried out (for example, setting default font for the document causes a relayout).

At least part of the work can be avoided by setting the page size of the document to QSizeF(0,0) in QDeclarativeTextPrivate::ensureDoc() because QTextDocumentLayout::documentChanged checks for the page size and skips layout if it isNull(). The proper size will be eventually set in the QDeclarativeTextPrivate::updateSize().

Patched tests/benchmarks/declarative/qdeclarativecomponent shows about 50% reduction in component creation time for rich text items.

Before:

RESULT : tst_qmlcomponent::creation():"RichTextLabel":
     0.67 msecs per iteration (total: 86, iterations: 128)

After:

RESULT : tst_qmlcomponent::creation():"RichTextLabel":
     0.29 msecs per iteration (total: 75, iterations: 256)