Describe the feature and motivation

For OpenCV 5, where we have a good chance to repair certain things, even if it slightly breaks API, it would be nice to replace some "home-brewed" names with somewhat more standard:

• uchar => uint8_t
• schar => int8_t
• short => int16_t
• ushort => uint16_t
• unsigned => uint32_t (where it makes sense)
• int64 => int64_t
• uint64 => uint64_t
• cv::String => std::string (some functions may take std::string_view for slightly better efficiency)
• cv::Ptr => std::shared_ptr

'int' is a special story - it's used very often in API and probably we should keep it as-is in API (except when int means int32_t, not just "integer with sufficiently large value range")

the old definitions may be put into a dedicated header that may be included by default in OpenCV 5 and not included by default in OpenCV 6+

Additional context

No response