[IMP] work on scrollable zone in web client

add a div parent to oe_view_manager_body, and move the searchview
drawer inside oe_view_manager_body.  This is in preparation to the
goal of making the oe_view_manager_body div scrollable
[IMP] slightly improve the html structure

of web client: remove an useless div .oe_view_manager_parent
introduced by a clueless developper.
[IMP] change scrollable area to main view

Now, only the main view is supposed to scroll (the part below the
header with the view selector).  The left menu, top bar and header
are supposed to stay fixed.  This is done by carefully using
position:relative and absolute.  However, this is likely to
introduce some (many?) issues, perhaps with tooltips position and
such.  The bug hunt is on!
[IMP] small css tweaks

correct a few problems introduced by the change to the scrolling area.

Note: body is in overflow:auto to allow scrolling when there is no view
manager (for example, client action)
[IMP] fix a css problem with multiple view_manager

when a view_manager is contained inside another, the
css rule for defining a scrollable zone (with position:absolute)
was also applied to the inside view manager.
[FIX] various css fixes related to scrolling

(1) it seems that position:relative behaviour is undefined in a td,
so it has been moved to the div below oe_application
(2) the left menu bar can now scroll vertically if necessary.
[IMP] improves the scrolling behaviour

the mail menu is a client action, so not subject to the same template
as regular views.  so, it had to be patched to properly scroll like
other views (meaning: the 'header', left menu and top menu are fixed)

Also, changes the width of the searchbbar into a min-width css property,
to allow it to grow more.  The reason is that we don't want the searchbox
to take two lines (or more), which will force the view_manager_header to be
higher, and which will cause slight issues with the scrollbar (absolutely
positioned)