Reorder tile layers after initialization

[aparshin]

Is it possible to control rendering order of layers after they were added to map?
Z-order of tile layer's container can be used for that, for example...

[mourner]

This will be addressed in the next version, quite a high priority task (and not only for tile layers, but for layers in general).

[mourner]

Decided to go with a more simple route for this task — added layer.bringToFront() and layer.bringToBack() methods, see commit 500848e