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20131231 (Tuesday, 31 December 2013)
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User interfaces get more pluggable
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I continued to remove hard-wired dependencies to ExtJS.  The process
is still not finished, but a next visible result is
:mod:`lino.projects.start` where the user can now easily switch
between the three (web) user interfaces that currently exist.

Before starting the following changes, I checked in the changes made
since :blogref:`20131227`.

Moved the following settings and methods from lino_site.Site to 
`lino.modlib.extjs.Plugin`::

    .build_admin_url()  --> .plugins.extjs.build_plain_url()
    .admin_prefix --> .plugins.extjs.url_prefix
    .build_extjs_url --> .plugins.extjs.build_media_url()
    .plain_prefix --> .plugins.plain.url_prefix
    extjs_base_url = None --> configure_plugin('extjs',media_base_url=None)
    extjs_root ="foo" --> configure_plugin('extjs',media_root="foo")

Test suite passes. Checkin.
    
TODO: 

- Possibility to log in from Plain ui and Pages ui.
- move get_css_includes and get_js_includes from ext_renderer to
  the extjs plugin
- Possibility to switch back from "Admin" to another ui
- error message if some of the old settings hasn't been removed?