[MacOS] RawTherapee retina font performance develeopements

With regards to the font size and default font performance on the Retina/HiDPI display, I have built from the dev branch which incorporates a codified font dpi, and a change to the default font options to prevent unusual behavior while choosing a font.

The issue:

The 5.2 dev build: https://filebin.net/fi94crziy0sv14po/RawTherapee_OSX_10.9_64_5.2-28-g665847db.zip

Version: 5.2-28-g665847db
Branch: dev
Commit: 665847db
Commit date: 2017-08-04
Compiler: clang-mp-3 3.9.1
Processor: generic x86
System: Apple
Bit depth: 64 bits
Gtkmm: V3.22.1
Build type: release
Build flags: -arch x86_64 -O3 -mmacosx-version-min=10.9 -std=c++11 -mtune=generic -Werror=unused-label -mmacosx-version-min=10.9 -flto -fopenmp=libomp -Werror=unknown-pragmas -Wall -Wno-unused-result -Wno-deprecated-declarations -O3 -DNDEBUG
Link flags: -arch x86_64 -mmacosx-version-min=10.9 -mtune=generic -headerpad_max_install_names -flto
OpenMP support: ON
MMAP support: ON