A mass commit to better use g.assertUi in several plugins.
In particular, added call to g.assertUi in bookmarks.py: without it, bookmarks.py can crash ungracefully in the Leo bridge because class FlowLayout depends on QLayout.
Added a standard docstring to most top-level init methods.  These methods *are* important.
Continued my policy of removing almost-never-used << version history >> sections.  Imo, git log message are sufficient.

Leo build: 20140918072326