Simplify Time Picker to only show multiples of 15min

[ltog]

When adding opening hours, there are two options to set the time:

1. Drag the drop-formed markers
2. Use the menu "Display time picker"

In this issue I'm refering only to option 2.

I'd like to be able to use a simplified time picker that is shown by default: This time picker would not show all minutes values from 0 to 59, but only 0, 15, 30, 45.

Further there should be a checkbox somewhere to enable the full time picker that is currently used if a special opening hours entry requires it.

This change would allow to enter opening hours a bit faster.

PS: I know that option 1 only uses multiples of 15min. However, since the physical length corresponding to 15mins is very small, I have trouble using it and therefore currently prefer option 2.