[sailfish-browser] Remove hard dependency to Silica Theme

Worth moving the preferences as static consts the beginning of the file as prefs are now used from multiple places. Something like: ```gPrefEmbedLiteZoomMargin```

Static memory footprint wise define would be better, could be like ```#define PREF_EMBEDLITE_ZOOMMARGIN QString("embedlite.zoomMargin")``` something like that (QLatin1String should do the job as well).