Add region categories

[vlachvojta]

Internal export: (pseudo PageXML)

• All regions are RegionLayout with category attribute (saved to XML as TextRegion element with category in custom attribute)
• Set OCR/OMR Engines to work only with some types of lines
• Set Layout Engines to work only with some types of regions
  Merging overlapping regions. (Text layout engine which detects region/line inside of other region, adds its lines the given region. Using geometry and coords to determine if some region/line is inside of some region) - not usefull feature

[vlachvojta]

Work In Progress in music branch.