Mergeable datasets

  - Merge method on DataCollection class
  - Merge action item in layer tree widget
  - Automerge suggestions on interactive load
Document data merging
Improve merge interface
Merge pull request #396 from ChrisBeaumont/merge

Allow dataset merging
ENH: specify hot keys for the majority of menu items

Allows for more convenient menu exploration/navigation/execution
Merge pull request #404 from yarikoptic/master

ENH: specify hot keys for the majority of menu items
Remove & from dict key (introduced in e252af49e47e73e0a84f1e52fb664177ca0f7dfa)
Add MaskSubsetState class

With the subset.state_as_mask method, any subset can be converted
into essentially a lookup table. This makes it easier to propagate
subsets to other datasets in some situations (for example spatially
overlapping datasets)
Add Menu item for maskifying subsets