Add support for showing only a single day

[RR28]

i tried to filter to show only 1 day using the method shown to filter the weekend but only hides the weekend , please try to fix it also tried to filter using only to show 1 using the filter method only hides the weekend

[tobiasschuerg]

This behaviour is intendet and not a bug. The idea is to always show the full week and only show the weekend if explicitely specified or if there are events on the weekend. Showing just a single days is currently not on my list.

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Still if you want to just view a single day, you could replace

https://github.com/tobiasschuerg/android-week-view/blob/a6e69dc56af4e3172fdf3b9e47f86b0fb9cf5001/library/src/main/java/de/tobiasschuerg/weekview/view/WeekBackgroundView.kt#L55-L58

by

        .apply {
            clear()
            add(DayOfWeek.MONDAY)
        }

and get