Code cleanup: Rename main implementation file from C to C++

The main purpose of this patch is to replace main.c with main.cc.

This step requires some #include directives of libxfce4util to be enclosed in extern "C" {} because some of the included Xfce4 C files are missing G_BEGIN_DECLS/G_END_DECLS macros. This issue has been fixed in libxfce4util's main GitLab repository, but (1) there hasn't been a release containing this fix and (2) it would delay xfce4-sensors adoption in conservative Linux distributions which are upgrading to newer Xfce library releases less often.

All C++ implementation files (.cc file extension) should include fixes.h before including any other header files.

See also: xfce/libxfce4util!19 (merged)

See also: !39 (comment 36108)