Correctly update symbol UI (units, transparency) when selecting a style

When selecting a style from the Saved styles list (Layer Properties -> Style), the UI is not correctly updated. For instance, if one saves a style with a certain transparency value, that slider is not set accordingly when selecting that style from the list.

Another separate but related issue is that the `clone()` function of a handful of symbol layers is incomplete, consequently when loading a saved symbol, the loaded symbol state won't always match 100% with the saved symbol state. I'll send a separate pull request for this.

Funded by Sourcepole QGIS Enterprise.