Modernize makefiles and remove dead code

This reverts commit 298f1ab5.

We're shipping libssp-0.dll now with mingw builds. We would
have enough time to test this for 2.5.0 now anyway, but we
also will most likely switch to MSVC for that release.

cmake/modules/ECMAddAppIcon.cmake is heavily patched to support sidebar icons, OUTFILE_BASE parameter and to not include 64 and 64@2x icons on macOS which are not supported. All changes are made in a way that we can upstream this.

It's needed since 586fd346 activated stack-protector-strong

Mainly uses target_include_directories instead of include_directories
so libraries public include directory get automatically added when adding
the target in target_link_library

It is unused. Also remove the corresponding error code

We use Qt's debugging code everywhere

This allow to remove all the csync time manipulation routne which are now unused

Use modern cmake with target_link_libraries and Qt5:: that
automatically add the include path and compile flags

There is no need to create a static library for the csync/std file, just
put everything together in csync

So that sqlite is not part of the public interface of csync
(and that the sqlite include path don't need to be passed when compiling
libsync or gui)

Use docker for Linux CI