st: Add support for font-feature-settings

Add support for font-feature-settings in CSS, and use it for tabular figures in a few places in the default theme.

Everything that Pango supports for font features should be supported by the font-feature-settings CSS property, as well as the CSS keywords normal and inherit. I didn't implement initial or unset because they don't seem to be supported elsewhere in this theme code.

I intend to apply tabular figures to all the UI elements that contain numbers in columns or are likely to have numbers that update frequently (which results in a visible ‘jitter’ effect when proportional figures are used). In this MR, those elements are:

• the entire top panel, on the grounds that it contains not only the clock which can update every second if seconds are shown, but also other widgets from extensions that may be showing numbers that change frequently, and the downside of overusing tabular figures is small;
• the numbers used in the calendar, which are presented in columns; and
• the clock on the lock screen, which again can update every second.

This may not be exhaustive; let me know if you think I missed anything.

Closes #34 (closed).