Fix bug that prevented avatars to be shown in groupchats. Bug was due to a wrong re-use of a local variable (local variabes are evil :-).

Additionally: Remove some appreviations and unneeded local variables.

don't show in roster / notification that a contact unsubscribed us if we already popup up the dialog. Fixes #5470

I remove this line because it makes no sense to assume that we have joind a groupchat, just because someone request a VCard for a jid which pretends to be from a Groupchat.

 * set default file type for avatars centrally
 * remove client side decision logic determing if a avatar loookup shall happen for a groupchat contact or for a normal contact
 * remove unused method (this one was triggering the traceback in #5461)

Showing it is more consistent than hiding it from the user. (We do the same in the Groupchat roster)

This option allows the installation of pyfiles in PYTHON/site-packages/gajim
instead of DATADIR/gajim/src which is a quality policy for some distributions.

Those variables were confusing because autoconf usually defines DATADIR to
PREFIX/share and gajim uses both defs.datadir=PREFIX/share/gajim and
gajim.DATA_DIR=PREFIX/share/gajim/data.

Now, autoconf will use PKGDATADIR for setting the (renamed) defs.basedir
variable which will be used to set gajimpaths['DATA_DIR'] and
gajimpaths['ICONS_DIR'] in configpaths.py

Gajim will also looks for the ${GAJIM_BASEDIR} environment variable instead of
${datadir} which was really too generic (no namespace).

use autotools tricks:
nobase_ prefix -> hiearchical installation
dist_ prefix -> include files in dist archive

It expanded version is GAJIM_SRCDIR.

It indicate where pyfiles will be installed (for now: PREFIX/share/gajim/src).

simplify src/Makefile.am by using nobase_ and dist_ prefix.
nobase_ allows hiearchical installation instead of flat directory
installation.