Always we see a warning about JavaScriptCore not being right version,
caused by main.py requiring WebKit yet viewhelp.py requiring WebKit2.

Seems to be no reason not to support WebKit in addition to WebKit2 in
the same code base.  Toolkit already supports both.

- remove gi.require_version from main.py,

- strip browser specific code from viewhelp.py,

- add new Browser base classes, one for WebKit, one for WebKit2,

- avoid GdkX11.X11Window.foreign_new_for_display if xid unknown,

- recognise which browser class to use based on OLPC OS release.