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>

man/weston.ini.man

@@ -212,6 +212,13 @@ output. Tile repeats the background image to fill the output.
 sets the color of the background (unsigned integer). The hexadecimal
 digit pairs are in order alpha, red, green, and blue.
 .TP 7
+.BI "clock-format=" format
+sets the panel clock format (string). Can be
+.BR "none" ","
+.BR "minutes" ","
+.BR "seconds" "."
+By default, minutes format is used.
+.TP 7
 .BI "panel-color=" 0xAARRGGBB
 sets the color of the panel (unsigned integer). The hexadecimal
 digit pairs are in order transparency, red, green, and blue. Examples: