Fix travis buildbot logic
State documentation
More state work to support session restoration
Tweaks for state saving
State saving for RGB mode in image widget
State polishing: slice restoration, scatter attribute syncing, range subsets
Label slices. Closes #222
Slice sliding preserves zoom. Closes #224
Added docstring
State load bugfixes for positions and tabs

Closes #221
Fix rebase mistake
Merge pull request #219 from ChrisBeaumont/state

State saving and loading