Bug#904879: Can not select predefined time format in clock widget

[elizabeth.peterson@protonmail.com]

This appears to be upstream bug #11527
 (https://bugzilla.xfce.org/show_bug.cgi?id=11527), which was fixed over several
 commits* in the git development repo post 4.13.

I took a stab at backporting the relevant patches, but discovered that they were
written after the GTK3 transition. I don't have the knowledge to easily rewrite
the validater code for GTK2, and merely applying the "clock: Fix invalid
datetime formats appearing in list" patch didn't solve the problem.

* Relevant upstream commits are:

e43248058e67e1cf2725788c894616dc431458cd clock: Fix invalid datetime formats appearing in list
a25624c894e93e1ff354f597975639ef0c2822c2 clock: Add a validator for digital custom formats
989d654b5d6b33e1c4127c740c806420a30b45a4 clock: Fix format validation