Remove GeoExt.tree.Panel and ensure drag & drop functionality by enhancing LayersTree-store

[marcjansen]

This PR suggests a larger refactoring of the tree classes. In particular the following is suggested:

• Remove GeoExt.tree.Panel and instead enhance the GeoExt.data.store.LayersTree to handle any possible drag/drop events
• Move static functions from GeoExt.tree.Panel to a dedicated utility class
• Remove the pointless showLayerGroupNode config from GeoExt.data.store.LayersTree

Additionally a bug is fixed: (we could not actually handle the store remove event before 36b8a81)

If you have been using the GeoExt.tree.panel previously, you now need to refactor your code from

Ext.create('GeoExt.tree.Panel', {
  // …
});

to

Ext.create('Ext.tree.Panel', {
  viewConfig: {
    plugins: { ptype: 'treeviewdragdrop' }
  }
});

Fixes #91

Please review. /cc @KaiVolland, @chrismayer & @AndersVittrup

The work in this PR has been done by @weskamm and me.

[KaiVolland]

I would prefer "onLayerCollectionAdd" and ...

[marcjansen]

Will rename, sure!

[marcjansen]

@KaiVolland as you suggested, methods are now renamed as you suggested

[vittrup88]

It fixes the issue. I also checked with the "length" of the layers. How many can you have. I earlier described it would overwrite and not show all layers after a give numbers of layers in the legend.
That is also working as it should now. I tried adding 120 kml files and no preformance problem and i can see all layers in the panel! Great work!

[marcjansen]

@AndersVittrup, cool thanks for the feedback

[KaiVolland]

All seems fine to me. Please merge. 👯

[marcjansen]

Thanks for the reviews and tests @KaiVolland & @AndersVittrup!

Again a hat-tip to the input of @weskamm