Improve formatting, adjust indenting for proper doxygen output.

Doxygen has changed in version 1.8.x, such that some (or all?) formatting
commands in lines with more than 3 spaces at the beginning of the line
are ignored (displayed verbatim). 


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10315 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Rename doxygen image file hello.C.png to hello_cxx.png.

Doxygen 1.8.x wouldn't show the image hello.C.png in its pdf output.
Maybe a doxygen bug - or feature ?


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10316 ea41ed52-d2ee-0310-a9c1-e6b18d33e121