[IMP] Add the ability to round timesheeted times (in minutes)

[StephaneMangin]

Add the ability to round time using a delta of minutes (max: 240)

Examples:
With a rounding of 15 minutes, tasks are rounded like this:

• less than 00:07:30 -> 00:00
• more than 00:07:30 -> 00:15
• 01:34 -> 01:30
• 02:45:01 -> 02:45
• 02:46 -> 02:45
• 02:52 -> 02:45
• 02:53 -> 03:00

Also a force rounding above option has been added to force time to be rounded up (except for start entries indicated by ***):

• less or equal to 00:00 -> 00:00
• less than 00:15 -> 00:15
• 01:34 -> 01:45
• 02:45:01 -> 03:00
• 02:46 -> 03:00
• 02:52 -> 03:00
• 02:53 -> 03:00