Fix non null variables at compile time

Fix contextual substitution

Fix ligature preview
Fixes bug 51

Merge branch 'master' of github.com:johanmattssonm/birdfont

Merge branch 'master' of github.com:johanmattssonm/birdfont

Rename text rendering directory

Merge branch 'master' of github.com:johanmattssonm/birdfont

Substitutions for space character

White space fixes

Skip empty ligatures in kerning display

Apply OTF substitution in kerning tab

Merge branch 'master' of github.com:johanmattssonm/birdfont

Fix ligature substitution in spaving and kerning tab

Run contextual ligature substitution before the non-contextual substitutions

Merge branch 'master' of https://github.com/johanmattssonm/birdfont into HEAD

Process contextual ligatures in kerning and spacing tabs

Thread safety in text rendering

Merge branch 'master' of gitorious.org:birdfont/birdfont

Conflicts:
	libbirdfont/BirdFontFile.vala

Fix carret position in text area

Add abstract table class

Add unassigned glyphs (ligatures) to ttf fonts and kerning tab.

Merge branch 'master' of gitorious.org:birdfont/birdfont

Switch to GTK3

Fix memory leak

Kerning by classes
This implementation uses the PairPosFormat1 to store kerning values in
TTF files and has a new kerning display that allows the user to define
kerning classes.

Some code to do PairPosFormat2 is included as well but it seems like
the PairPosFormat1 is the better solution.