Layout enhancements to the clock plugin and the panel

Ghost Userrequested to merge
(removed):clocks into master

Rework of layout mechanism in clock plugin and its widgets: 473974b9. Replaced fixed size requisition with Gtk3 layout mechanisms. This commit also introduces size clamping for LCD and Binary clock widgets.

Depends on https://gitlab.xfce.org/andrzejr/xfce4-panel/-/commit/31354407305abf325321fd63311e8cded982b31a and https://gitlab.xfce.org/andrzejr/xfce4-panel/-/commit/721a01361fb90ab12c352458b350eef961d24cc2 enhancements to itembar and convenience functions in libxfce4panel (no API change).

https://gitlab.xfce.org/andrzejr/xfce4-panel/-/commit/271f3c5e7be3ba7de41376fa607aab4d7cf5c711 Improves digital clock layout in larger horizontal panels.

https://gitlab.xfce.org/andrzejr/xfce4-panel/-/commit/1b59aaab568cb312d29a24a455273bede0be8f41 Simplifies layout of the LCD clock widget, so that it the panel layout does not vary in time.

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