[gtk] Drop use of GdkPixmap

Part of #13.
[gtk] Do not include individual headers

Part of #13.
[gtk] Don’t use deprecated symbols

Part of #13.
[gtk] Use accessor functions

Part of #13.
[gtk] Remove commented out code
[gtk] Remove needless use of GTK_OBJECT macro

Part of #13.
[gtk viewer] Port to GtkBuilder

Which gets rid of the deprecated GtkItemFactory.

Part of #13.
[gtk viewer] Don’t use deprecated GtkFileSelection

Part of #13.
[gtk] Port to GTK 3

Fixes #13.
[gtk] Don’t use deprecated GtkStyle
[gtk] Drop a bunch of deprecated symbols
[build] Simplify this
[backend] Move common code to the base class
[backend] Move more common code to base classes
[backend] Move more common code to base classes
[data] Auto-generate entitiesTable.inc again

Now using htmlmathml.json file from:
  http://www.w3.org/2003/entities/2007doc/#htmlmathml

Fixes #11 as a side effect.
[data] Remove unused entities-table.* files
[build] Add missing XML_LIBS for GTK viewer
[build] Use AM_CPPFLAGS

We no longer need individual _CPPFLAGS after we dropped the different
dll export defines in 9cb4ce5.
[build] Remove unused top targets