Remove Calendar extensions (+ add JavaTime extensions)

[Del-S]

kaal/kaal-core/src/main/kotlin/cz/eman/kaal/core/util/CalendarExtensions.kt

Line 18 in b2b2183

this.set(Calendar.HOUR, 0)

Should be HOUR_OF_DAY instead of HOUR since HOUR is AM/PM 0-12 (12 hour format). So if you are in PM you will actually get 0PM which is 12 hours in 24 hour format.

Since Calendar has its problems and is pretty old we should migrate to Java Time. Thus remove Calendar and provide Java Time.