vdx: optimize VDXRenderer::draw_polygon() to combine fill and stroke

VDX can have fill and/or stroke with a single shape by just setting the
right properties. Also now stroked polygons are staying polygon rather
than becoming polyline.
vdx: optimize VDXRenderer::draw_ellipse() to combine fill and stroke

VDX can have fill and/or stroke with a single shape by just setting the
right properties.
dox: [warningectomy] missing parameter documentation ...

 - add plug-ins/pdf for documentation extraction
 - remove second occurrence of plug-ins/svg
 - fix doxygen complaints about missing parameter documentation
 - fix a bug in export-render.py where the parameter was documented,
   used in the implementation but not declared
 - fix title mismatch for group ObjectConnects
Bug 606377 follow-up: beautify filter names and fix help for --list-filters

Now that dia --list-filters shows all the available export filter strings
replace some abbreviations with the full name and shorten the longest
filter name to fit typical terminal width.
Bug 735430 - Implement VDXRenderer::fill_arc()

by copying draw_arc() implementation, changing to fill and adding two line
segments. To get even nicer appearance of boxes with rounded corners an
implementation of VDXRenderer::draw_rounded_rect() would be needed. But
that's an enhancement left for another time.
vdx: fix position of bottom aligned text

Not shifting by height makes text position right for text_tests.vdx
Doing no shift for the other VerticalAlign screws them ...
dox: improved documentation for renderers and importers

The DiaRenderer interface is now completely documented with renderer
implementors in mind. To support sub-grouping in the documentation some
code needed to be moved around, but there is no functional change.

Also more importers are included with the Doxygen generated documentation.
layer-widget: Add edit name button to button row

This makes the layer list slightly wider, so it is loosing it's
scroll-bar for most diagrams I looked at.