Ernst load order

Comes with https://github.com/nens/lizard-nxt/pull/414.

Situation
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We used to have a hardcoded mechanism to remove the utfgrid from leaflet when the png's were loading. This was crappy but made the app feel a bit faster.

Problem
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It was hardcoded and did not fit in the refactored layer, layergroup etc. 

Solution
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Add load order to the layers. When toggling layergroups, turn layers on by their load order. So if everythng works well now:

When toggling an imaginary layegroup with layers:
1. loadOrder: 3
2. loadOrder: 3
3. loadOrder: 1
4. loadOrder: null

It will first load layer 1 and 2 simultaneously. Then layer 3. And then layer 4. 

When moving the map it will turn layer 3 and 4 off. And when layer 2 finishes, layer 3 and 4 are turned back on.

Additionally
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- [x] Layer choosers are toggled 3000 ms after the app loaded to speed up initial load significantly.