Update to new upstream release 5.2.

Changelog: http://rawtherapee.com/blog/rawtherapee-5.2-released
Detailed: https://github.com/Beep6581/RawTherapee/commits/5.2

• Add a "remote" mode (-R command line option)
• Add DCP profile for FUJIFILM X-T20
• Overall speedup and stability improements, fixed memory leaks
• More accurate tone mapping tool preview