Add setting to toggle 24-hour time format for the chat

[davidebeatrici]

By default QTime uses the 24-hour time format when converting to a string.

This commit adds a setting (a checkbox) which allows users to toggle the format.

The setting only applies for new messages, the already shown ones will retain the previous time format.

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Fixes #3823.

[Kissaki]

The OS has and provides information of regional and formatting settings. We should use that instead of duplicating those settings.

When was this issue introduced? 1.3.0? Or afterwards in the snapshots?

[davidebeatrici]

The OS has and provides information of regional and formatting settings. We should use that instead of duplicating those settings.

I initially used Qt::DefaultLocaleShortDate, which is what we currently do for the date, the issue is that seconds are not shown.

With Qt::DefaultLocaleLongDate seconds are shown, but the timezone (e.g. "UTC") appears as well.

QLocale::system().timeFormat(QLocale::ShortFormat) returns h:mm AP.

QLocale::system().timeFormat(QLocale::LongFormat) returns h:mm:ss AP t.

When was this issue introduced? 1.3.0? Or afterwards in the snapshots?

According to #3823 it appeared with Mumble 1.3.0.