Rename PropertyGrouping object to ItemGrouping and move YearGrouping

The 'Line' object concept tree introduced in 0d775cf2
mistakenly called this object PropertyGrouping,
but in reality it's not about the source of the grouping
but about its contents: in this case an Item page.

Indeed, the methods in that object are only about
formatting the grouping link in case of ItemPage,
and about formatting a higher-grouping, a concept
that only makes sense for Item groupings.

Also move YearGrouping straight under Grouping,
which tellingly does not require any change in tests.

The new tree is thus:

AbstractLine
└── Grouping

├── NoGroupGrouping
├── ItemGrouping
├── YearGrouping
├── UnknownValueGrouping
└── TotalsGrouping

(there could/might be a need in the future for
a level above Item and Year, but not for now).