Rev 5558: changes to leoGlobals.py and the startup sequence

[Edward K. Ream]

Rev 5558 contains proposed changes to Leo's startup sequence. This might cause problems with some plugins, but the old code that temporarily bound g = nullObject is wrong: it hides problems.

Please report any problems immediately.  If you do experience problems, setting new_load_g = False at the start of leoGlobals.py should fix things.

Here is the checkin log:

QQQQQ
More work on making leoGlobals.py usable stand-alone. All unit tests pass with Python 2/3.

- Added g.new_load_g = True.
  When True, leoGlobals.py sets g at end and runLeo.py does *not* set leoGlobals.g.

- When g.new_load_g is True, g is None while leoGlobals.py is being scanned.
  This may cause problems for plugins, but setting g = nullObject is wrong.

- Moved @g.command('open-url-under-cursor') to leoEditCommands.py.
  The @g.command decorator may not be used in leoGlobals.py.
QQQQQ