There is a new check box that allows the user to tell whether a map should
keep the layer styles (it will store the state when the check box is checked).
The stored layer styles keep a snapshot of each layer's configuration
instead of just keeping name of the style.
This solves issues with styles and visibility presets in composer which were
not completely compatible.
This code has been funded by Tuscany Region (Italy) - SITA (CIG: 6002233F59)
and commissioned to Gis3W s.a.s.
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