Rename 'LoadFromFile()' and 'SaveToFile()'

Rename 'LoadFromFile()' into 'Load()'.
Rename 'SaveToFile()' into 'Save()'.
Add 'BitmapFileHeader::ReadFromFile()'
Add 'BitmapInfoHeader::ReadFromFile()'

Reorder definition of 'GetBase'.
Add 'BitmapInfoPtr::ReadFromFile()'
Simplify 'BitmapFile::Load()' and 'ReadFromFile()'
Add 'BitmapFile::IsValid()'
Add 'BitmapFileHeader::WriteToFile()'
Add 'BitmapInfo::WriteToFile()'
Simplify 'BitmapFile::Save()' and 'WriteToFile()'
Change 'BitmapInfoPtr' from 'struct' to 'class'
Change access level of '(ReadFrom|WriteTo)File's

from 'public' to 'private'.