faces: follow HIG recommendations on capitalization

The HIG now contains specific guidance on capitalization; broadly speaking, labels for most controls should use sentence capitalization. The sensor face cofiguration pages are currently rather inconsistent with regard to this, both across the different faces and sometimes even within one face.

This change makes all the face configuration pages in libksysguard follow the HIG recommendations, and adds/expands translation context for the affected strings. In addition, there are a few small edits to the existing labels to make them more consistent with the recommendations on label lengths; in particular, the unnecessary plural is dropped from "sensors legend" and the somewhat cumbersome spinbox label "Amount of history to keep: [60 seconds]" is shortened to "Show last: [60 seconds]"

Example screenshot comparison:

before	after