Update stopwatch granularity on change in settings

Summary:
Bug: 349861

[This is my first commit to KDE, so bear with me if something is wrong. Thank you.]

Steps to Reproduce:

1. Open Kronometer
2. Show only tenths of seconds from the settings
3. Start the stopwatch
4. Show also the hundredths of second from the settings

Actual Results:
The hundredth digit is stuck to the previous digit.

Expected Results:
The hundredth digit should be updated every 1/100 of second.

The same happens with milliseconds.

Test Plan: This is a simple basic bug so test plan not required.

Reviewers: elvisangelaccio, Kronometer

Reviewed By: elvisangelaccio, Kronometer

Tags: Kronometer

Differential Revision: https://phabricator.kde.org/D22290