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
some more help for installing from source

Related to the discussion in https://github.com/fontforge/fontforge/issues/153
Require Python >= 3
Fix lots of compiler warnings

Mostly unused variables and parameters
.dir-locals.el: actually make it all work

Since I use dtrt-indent, I hadn't noticed this wasn't working at all;
sorry. Actually tested now.
configure.ac: fix some whitespace in generated config.h
parsepdf.c: fix a comment typo
psread.c: fix most compiler warnings
namelist.c: fix an annoying number of compiler warnings about const
Change some member names to avoid clashes with re-#defined symbols
Make command-line Python scripts more convenient
Overhaul tests

Add more details of files needed for tests

Add some files that are missing but can be put into git (i.e. are
suitably licensed)

Improve test descriptions

Fix tests/Makefile.am for 'make distcheck'
python.html: small improvements