When a grouping per date is done, per default the granularity is one month. This patch increases the granularity for the last month:
- "Weeks" are used for the last 4 weeks excluding the current week.
- "Days" are used for the current week.

Additionally some cleanups of the code has been done, as the method DolphinModel::data() was too long already.

No backport will be done as new strings are introduced. The patch has been reviewed by Rafael Fernández López.