Highlight pre-translated segments of bad quality / missing translations in visual translation

For visual translation jobs there will implemented

•  segments pre-translated by MT according to their match rate value
• all segments that are still in the source with the color for the worst match rate range
• what highlight-color is chosen for what match-rate range is defined in Zf_configuration
• a toggle button in the layout area will switch the high-lighting on/off
• as soon as a segment is edited, the highlight is removed, because now the processing state is not the initial one any more (here the processing state pre-translated is still missing, see TRANSLATE-1643; Please discuss with Aleks an intermediate solution)