Fix moved Pandas class

Closes #383
Variables dragged into terminal take default label from .label attribute

Closes #392
Mergeable datasets

  - Merge method on DataCollection class
  - Merge action item in layer tree widget
  - Automerge suggestions on interactive load
Document data merging
Merge pull request #393 from ChrisBeaumont/pandas

Fix moved pandas class
Merge pull request #394 from ChrisBeaumont/terminal_label

Better default terminal variable names
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)
Layouts don't overlap
Preset layouts
New layout snap algorithm in GUI