Remove Screenwidth from default theme

Fix issue #288
Make defaultbutton and cancelbutton grey
White background instead of light grey
Fix a typo
Revert to original colours in CharView

The current theme has softer tones that are nicer to look at but too low
in contrast.
Removing hotkeys from tango resources file
Adding hotkeys back
Import Yuri's screenshot to use by desktop/fontforge.appdata.xml

Removed extra </p>, then moved other </p>s
Includes of non-fontforge headers should use angle brackets, not quotes
Remove some free-ing code previously missed, which was causing a crash
Merge pull request #1294 from rrthomas/master

Includes of non-fontforge headers should use angle brackets, not quotes
Implement fontforge_calloc veneer using xzalloc: seems to fix some crashes
search.c: fix warnings and shorten a little code
Allow libgc to instrument pthread_create
gio.h: remove some dead declarations
splineutil2.c: rename a variable called 'bool' to 'flag'
Use proper stdbool.h booleans
Fix crashes with moving splines

This arose because SplinePoint data was being held in glib lists, which
were not scanned by libgc, and so the SplinePoints were assumed to be
garbage, and collected.

The fix should have been to make glib use libgc, but the ability to
change the allocator was broken in glib 2.36:
https://bugzilla.gnome.org/show_bug.cgi?id=701694

Hence, we have to avoid storing data blocks allocated by libgc in glib
data structures.

Hence, use gnulib lists instead.
Merge pull request #1299 from rrthomas/master

Fix various libgc-related crashes
Remove true and false #defines