Make sr-lock-window a bit more resilient

If I type C-x 1 in the viewer window, make Sunrise Commander think that
it was exited.
Updated version to 420
=Bugfix: tabs aliases were lost after a call to `sr-tabs-transpose'
+Slightly improved renaming of tabs: an empty alias now resets the label to its default value.
=Bugfix: prefer `labels' over `flet'.

In latest versions of Emacs, `flet' is not working as reliably as it used to.
=Minor refactorings.
+Improved path highlighting mechanism.

Do not create overlays for paths in the passive pane, instead set the
passive path face everywhere from the beginning (using font-lock in
Sunrise core and text properties in the Tree extension) and modify it
as needed.
Add possibility to disable automatic killing of unused buffers.

Manually managing Sunrise buffers allows to maintain marks, narrowing, or any
other modifications made to a buffer, even if that buffer loses focus. This is
the default operation model offered by Dired and some long time Emacs users have
learned to take good advantage of it.
+New implementation of history navigation.

The original implementation of history navigation worked quite well most of the
time, but not always. The ring structure caused some of the directories recently
visited to get "lost" under certain conditions. The new implementation is stack
based and strives to maintain the most recently visited entries as near as
possible to the currently displayed entry.
+Improved active/passive path graphical highlighting.

There is no need to generate overlays to highlight the paths in both the active
and the passive pane. We can instead use font lock to first display the path in
both panes with the passive directory face, and then change the appearance of
the path in the active pane using a dynamically built overlay.

Additionally, some times there is a need to use more than one face to display
the active path, e.g. when using simultaneously wdired and the mirror extension
to rename files inside a compressed archive, SC needs to remember both faces and
restore them in order: default face -> mirror face -> wdired face when "going
in", and the other way around when the whole operation is completed. The current
implementation replaces the previous `sr-current-path-face' plain variable with
a list `sr-current-path-faces' to achieve this effect.
+Bugfix: `sr-virtualize-pane' should effect only the active pane.

When both panes displayed the same buffer, `sr-virtualize-pane' used to make
both panes virtual, which was not the desired behavior.
=Make unionfs-fuse the Union FS implementation by default.

It does have issues, and in some cases its still preferable to use funionfs, but
the latter seems to be not maintained since 2007, and Debian has removed it from
its package repositories.
+Use lexical bindings.

The "lexical-binding: t" declaration has been added to the main script and to
all the extensions (excepting for the "old checkpoints" for Emacs 22), and all
the compiler warnings have been addressed (at least those from Emacs 24.1.50.1)
=Minor bugfix: refresh modeline (if present) in synchronized tree mode.
The Sunrise Commander -- version 6
=Minor bugfix in `sr-beginning-of-buffer': do not use `while' to move over '.' and '..'