gtk_orientable_get_orientation now throws warnings so remove it. It does not appear to work anyway.  Applet tested in a vertical (left side) panel as well as a horizontal one, as long as the vertical panel is wide enough to accept the applet it works and puts the frequency and units on two levels. Puts them on one level in a horizontal panel.

and require mate-panel >= 1.17

WARNING: use GTK+3 build of libmateweather for this build.
that lib is not migrated to GTK+3 only as we will possibly
use libgweather instead of it.

And bump polkit requirement.
polkit_authority_get is deprecated since polkit-0.97 from 2010.
So the requirement bump should be save for all distros.

there's implicit declaration warning with or without it,
see https://bugzilla.kernel.org/show_bug.cgi?id=153161

Stop BlackMATE, possibly other themes from cutting off the end of the units label and get rid of an #1f block for GTK 3.19 at the same time.  GTK 3.19 seemed to pad the end of this label a bit, not so with released GTK 3.20.

In gtk3,  replacing "label" with "applet->label" makes the gtk_widget_get_preferred_width functions work for the frequency and unit labels without issue. Trim them a bit in gtk3.19 builds as they render 2px wider there for some reason. The exception is the percentage label. That one apparently always starts with "0%" entered-and it does NOT update it's width when larger character strings come up. Thus, a fixed figure of 20px still has to be added to that one. On the other hand, that will ensure the applet never jumps from percentage changes. Height of the whole applet is still limited somewhere to 24 px, still looking for that issue as it hurts the match to taller panels.