SignalObserver: fix further refcounting issue in signal observer

add proper refcounting to XML SignalObserver. not refcounting caused crash upon opening Filter Editor dialog for the first time with a filtered object selected.

syntx of null pointer dereference checks

C++ification of SPObject continued along with the onset of XML Privatisation. Users may checkout [grep -Ir XML Tree *] in the source code and all the places where the XML node/Tree is being used shall be reflected.

Now users can design a font within inkscape, save it and then open the SVG file in Fontforge in order to export a truetype font (or other system font formarts fontforge supports). This improves previous workflow of font design using Inkscape which involved creating one SVG for each glyph. Now user only needs to create a single SVG file containing an SVGFont. Glyph kerning settings for the font can also be defined withing Inkscape itself with live preview. The kerning management still needs some improvements but is currently functional at least. Improvements in the SVGFonts dialog: * In Global Settings tab you can define the font family name. Other attributes should be added to this tab in the future. * Glyphs tab allows the user to: ** see a list (combobox) of glyphs available in the currently selected font. ** add/remove glyphs ** edit glyph name and unicode ** set the glyph curves based on a given path (selected from canvas). Same feature for the missing glyph. * Kerning tab allows user to: ** add new kerning pairs ** adjust kerning values of selected kerning pair ** live preview while adjusting the kerning values Code refactoring: * Inner classes DocumentProperties::SignalObserver and FilterEffectsDialog::SignalObserver were duplicated code and another instance would be needed in SVGFonts dialog. So, I moved it to Inkscape::XML::SignalObserver (in helper-observer.{cpp,h}) * changed SPGlyph->glyph_name and SPGlyph->unicode from char* to Glib::ustring * added sp_remove_resource to the release method in sp-font.cpp * glyph curves used to be stored (in d attribute) and rendered upside-down. Now that bug is fixed. Sorry about this huge commit. I got one week away from the Internet during a xmas travel. The lack of 'net connection made me work more intensely in Inkscape :-D Felipe Sanches