desktop-shell: make panel clock configurable

This patch enhances the panel clock by adding a config file
option which can be used to either disable the clock or make
it also show seconds in the current clock format.

v2: Implement suggestions from Pekka:
    - Include Signed-off-by
    - Coding style fixes
    - Implement clock widget allocation by using
      width from cairo_text_extents
    - Highlight config option values in man page
v3: Implement suggestions from Pekka and Bryce:
    - Use CLOCK_FORMAT_* instead of FORMAT_* in the enum
    - Switch to using fixed clock widget size instead
      of one returned from cairo_text_extents
    - Fixes to config option highlighting in the man page
v4: Implement more suggestions from Pekka and Bryce:
    - Improve patch changelog
    - Move the check for CLOCK_FORMAT_NONE into the
      caller function
    - Fix a memory leak in panel_create introduced by
      previous revision of this patch

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57583
Signed-off-by: Armin Krezović <armin.krezovic@fet.ba>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/weston.ini.in b/weston.ini.in
index dff9e94..14a4c0c 100644
--- a/weston.ini.in
+++ b/weston.ini.in
@@ -7,6 +7,7 @@
 background-image=/usr/share/backgrounds/gnome/Aqua.jpg
 background-color=0xff002244
 background-type=tile
+clock-format=minutes
 panel-color=0x90ff0000
 locking=true
 animation=zoom