0028441: Coding Rules - move out nested Image_PixMap::ImgFormat enumeration to dedica...

0028441: Coding Rules - move out nested Image_PixMap::ImgFormat enumeration to dedicated enum Image_Format

Enumeration Image_PixMap::ImgFormat, previously declared as nested
enumeration within class *Image_PixMap*,
has been moved to global namespace as Image_Format following OCCT coding rules.

The enumeration values have suffix Image_Format_ and preserve
previous name scheme for easy renaming of old values.
E.g. Image_PixMap::ImgGray become Image_Format_Gray.
Old definitions are preserved as depreacated aliases to the new ones.

# Conflicts:
#	dox/dev_guides/upgrade/upgrade.md