[BUG] support non-numbers nicely as duration for Timed Effects in Initiative tool

[dthv]

Timed Effects in the Initiative tool are designed to support numbers that decrease over time until the effect vanishes. They actually do support permanent effects by typing in a non-numeral value into the duration field, which is, in principle, desirable.

However, if the entry in the initiative list gets to be active, normally, a numeral value would decrease by one (or vanish, if already zero). In case of any non-numeral value, the value changes to "NaN" (which probably stands for "Not a Number"?).

To be able to differentiate between e.g. permanent effects and effects that don't have a specific duration in time (e.g. with the duration of an "encounter" etc.), it would instead be desirable that the non-numeral value would simply be kept.