Improving text node updates

[uwerat]

There is no need to rebuild the glyph nodes in situations, where only the text colors have been changed.

This is already supported by QskPlainTextRenderer::updateNodeColor, but had been disabled in QskSkinRenderer::updateText. The reason for this was related to running into wrong update decisions in early stages of the project. In the meantime it might be obsolete as the concept for having very detailed dirty masks had been dropped.