Add support for timestamp format "HH:mm:ss,fff"

[chausner]

Changes proposed in this pull request:

• Add support for timestamps in format HH:mm:ss,fff

Rationale: timestamps with a dot as decimal separator (HH:mm:ss.fff) are already supported. However, in ISO8601, the comma is the recommended decimal separator:

[...] the decimal fraction shall be divided from the integer part by the decimal sign specified in ISO 31-0, i.e. the comma [,] or full stop [.]. Of these, the comma is the preferred sign.

Our software uses HH:mm:ss,fff today and we would prefer keeping this format since it's recommended by ISO rather than changing it for compatibility with Tailviewer.

Any expected problems concerning backwards compatibility of existing plugins?
No

Any expected problems concerning backwards compatibility of existing user settings?
No

Does this break existing user workflows?
No