Remove invisible layers from legend

[arnaud-morvan]

Description

Do not show invisible layers in composer legend.
Useful for raster layers as QgsMapHitTest only take care of vector layers.

Related with #14194 and #11641
May be backported to 2.18 before closing the tickets.

Checklist

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[wonder-sk]

Can we move the condition outside of the Q_FOREACH cycle? As far as I understand, the filterLegendNodes() method is called per each map layer separately, so we can leave early without looping through all nodes.

[m-kuhn]

Concerning backporting: is this considered to be a bug?

[arnaud-morvan]

@wonder-sk : yes, this is only called with nodes from the same layer. I've move the test outside of the for loop.
@m-kuhn : This is a missing feature that can be considered a bug. Backporting make sense.

[arnaud-morvan]

@wonder-sk : How about merging this ?

[wonder-sk]

Thanks @arnaud-morvan !

[rldhont]

@arnaud-morvan can you backport this fix to release-2_18 ?