New style option: "page break before"

[sbidoul]

Allow forcing a page break before a KPI.

Not sure yet if it should be a style option or a flag on the KPI.

[github-actions]

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[hbrunn]

In OCA/account-financial-reporting#1076 I now propose to introduce a split_after flag on KPIs that causes a new table to be rendered after a KPI with the flag set (why not in styles? Because then we can't share styles between non-splitting and splitting KPIs, and I think KPI is the right level of abstraction anyways).

Works quite nicely for my horizontal use case, but in this version I made it general enough that you can split your KPIs to any amount of tables. Page breaks should be a simple followup then.

Would you still prefer a patch on mis_builder, or should I start with mis_builder_horizontal to use as incubator for this feature? (I prefer that)