Tiled Gallery: Fix square layout remainder margin.

Remainder would be short in width from the regular rows by exactly the margin. This is now fixed by not substracting the margin from the remainder size.
Tiled Gallery: Bottom margin fix.

The root cause of why the bottom/top margin between rows in Tiled Galleries is twice as wide as between columns turned out to be specific to the mosaic layout, where the rows actually had a concrete 2px bottom margin. If we remove this margin, we have a clean equal margin everywhere.
Merge pull request #500 from rase-/margin-fix

Tiled Gallery margin fixes