[cleanup] reduce code duplication for integrated UI

 - there is no need to have two global _ui_managers
 - without these we have less conditional code
 - quite some global variables were just used to transport
   information in a single function
Bug 573261 - Avoid font kerning problems with pangocairo(fc)

There is a recurring bug with pangocairo related to kerning and font scaling.
See: https://bugzilla.gnome.org/buglist.cgi?quicksearch=341481+573261+700592
Rather than waiting for another fix let's try to implement the ultimate work
around. With Pango-1.32 and HarfBuzz the kludge in Pango is gone and apparently
substituted with a precision problem. If we now use huge fonts when talking
to Pango and downscale these with cairo it should work with all Pango versions.

Also add a sequence diagram used to understand the problem.
Bug 696438 - Make integrated mode the default (introduce --classic switch)

Still no UI option to toggle, for that see  bug 588208
Remove Edit/Layers from display menu

and make the integrated-ui menu more similar, e.g. add ExtionsStart(s)
Transport xmlError information via DiaContext

where appropriate. With diagram files these get visible in the UI.
For other xml files they are converted to g_warning() or message_error.
More PyErr_Warn(), less g_warning()