build: Updated Docker Linux 64bit image to Qt 5.12.6 and LLVM/Clang 9…

….0.0

* updated to CentOS 7
* removed 32bit image

start working on the next quarterly release

ui: Show Sourcetrail license on top in licenses window

* updated versions and license files of some 3rd party libraries

new quarterly release

new quarterly release

new quarterly release

new quarterly release

new quarterly release

Today you are making the best proposals