exr: write support and more

- Added support for writing EXR files
- Large image support (tested with 32GiB 16-bit image)
- OpenEXR multithreaded read/write support through the use of line blocks
- Build option to enable conversion to sRGB when read (disabled by default)
- Multi-view image reading support
- Options support: Size, ImageFormat, Quality, CompressRatio
- Metadata write/read support via EXR attributes

The plugin is now quite complete. It should also work on KF5 (not tested): if there is the will to include this MR also on KF5, let me know and I will do all the necessary tests.