Untranslatable strings and formatting of the raster min/max label

Originally proposed in pull request 757: https://github.com/qgis/Quantum-GIS/pull/757

The patch fixes a couple of previously untranslatable strings and provides a formatted string for the entire min/max label instead of concatenating it from 3 strings. Comes handy in many eastern languages in case the translator would like to add extra words or reorder the parts.