Styles refactoring

[sbidoul]

Styles are currently defined on

• report template: apply to the whole report
• kpi (aka rows): inherit report level style
• account detail rows: inherit report level style

Style are inherited with an override mechanism when they are merged.

A more extensive approach could be this style inheritance order

• template (mis.report): global style for the template
• (*) report (mis.report.instance): override global style per report
• (*) column (mis.report.instance.period): override global style per column (e.g. to set a color on comparison columns, ...)
• (*) subkpi column (mis.report.subkpi): override column style per subkpi column
• row (mis.report.kpi): override global an column style per row (e.g. to highlight subtotals)
• (*) subkpi cell (mis.report.kpi.expression)

(*) not implemented yet