Fix `distcheck' complaints.

Fix `distcheck' complaints.

  * Have `disobedience/Makefile.am' remove the images and desktop file
    it installs.

  * Remove `lib/versionstring.c' as well as its header file during
    clean.

  * Move source files (but not headers) from `lib/' to new `common/'
    directory if they're needed by other directories, and refer to them
    via the `VPATH' feature rather than using explicit pathnames.  (It
    doesn't work to have the whole of `lib/' on the VPATH because there
    are files in some of the other directories with the same names,
    which breaks in-tree builds.)