Keep current time position when changing week

In the week view, when current week is displayed, view is automatically set to the current time but when week was changed to something else than current week, view was always starting from 0:00. Now it will keep displaying the same time as was set previosly, so when user will move e.g. from current week to the next one, it will still see the same time range by default.

Closes-issue: #75 (closed)