Use tabular numerals in Digital Clock widget

Reason for the change

This was originally reported to the Inter font project as a bug, but is actually to do with Plasma's behaviour here. Tabular numerals have consistent widths, meaning when a number changes, it doesn't cause layout shift. In the case of the digital clock widget, that can mean moving the system tray once per minute, or even per second. This probably hasn't come up before because Noto, Plasma's default font, uses tabular numerals by default. Inter on the other hand supports tabular numerals, but doesn't use them by default.

Test plan

Set the Digital Clock widget to use a font which supports, but doesn't default to, tabular numerals, such as Inter, and optionally set "Show seconds" to "Always" to make the effect much more obvious. With tabular numerals, the layout shouldn't shift.

Screenshots or screen recordings

Before	After
before	after