uzbl-tabbed: simple support for named sessions (presets)

Create new flags, -l and --load-session. The argument is a filename in
config['saved_sessions_dir'].

Fix the 'preset list' command so the javascript alertbox shows up
correctly.

Create a new 'preset open' command which will open a new uzbl-tabbed
window with the requested session. Just like '-l', the argument is a
filename (no path). I happen to prefer using 'go' for this command
instead of opening a new tab.

Unlike the previous code, all the session files (even the 'default' one)
are supposed to be inside config['saved_sessions_dir'].

To test, try
    make TABBED_EXTRA_ARGS='-l sessionfilename' test-uzbl-tabbed-sandbox

    There is a problem with this testing, though: if you open more
    uzbl-tabbed windows via the 'preset open' command and then close the
    original one, the script will continue and close the event manager.
    It works fine when installed, though, since the event manager is not
    bothered.
Do not run if the old session file is still there (ask for user intervention)
uzbl-tabbed: add flag to list the named sessions
uzbl-tabbed: allow 'backup session' when loading
uzbl-tabbed: add 'backup session' command
uzbl-tabbed: the 'preset list' command works
uzbl-tabbed: better implementations for preset commands

use dmenu to interact with the named sessions, uzbl-tabbed itself
provides the listing

the command-line flags -L (to list) and -l (to load) continue to work as before

steal the 'go' binding (event NEW_TAB) to use as the 'preset open'
command. 'gn + ENTER' still exists, though, so no need to whine about
it.