Fix Makefile problem with make v3.82

* make v3.82 (and make v3.80) didn't handle pattern rules containing
wildcards (see #637505). So the rules for making POT files are changed,
now using GNU make's "Secondary Expansion" feature.
(bugfix by Miroslav Šulc <fordfrog@fordfrog.name>)
* Fix "make pot/gimp.pot".
(bugfix by Miroslav Šulc <fordfrog@fordfrog.name>)
* Fix (and use) image prerequisites for pot files (if an image is
changed, the pot file is updated).
Fix making pot files with image prerequisites
Don't mix implicit and normal rules

Mixed rules don't work with make-3.82 (bug #639057).
Suppress some harmless warnings
Disabled due to immaturity of the translation
Enable to use floating images in HTML

Add an XSL stylesheet that passes 'condition="float"' attributes
of <inlinemediaobjects> or <mediaobjects> without <caption> as
class attributes to HTML. Modify CSS stylesheets to make those
images float.

Conflicts:
	stylesheets/plainhtml.xsl.in
Preparing 2.6.2 release