Fixes related to tours

[astrofrog]

This PR:

• Improves the save dialog for tours
• Adds a test to ensure backward compatibility with old session files
• Preserves the original unmodified longitudes as an additional column in 3D mode

[codecov[bot]]

Codecov Report

Merging #73 into master will increase coverage by 0.89%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   72.64%   73.53%   +0.89%     
==========================================
  Files          18       18              
  Lines         859      869      +10     
==========================================
+ Hits          624      639      +15     
+ Misses        235      230       -5

Impacted Files	Coverage Δ
glue_wwt/viewer/tools.py	85.1% <ø> (ø)	:arrow_up:
glue_wwt/viewer/layer_artist.py	100% <100%> (+100%)	:arrow_up:
glue_wwt/viewer/state.py	100% <100%> (+100%)	:arrow_up:
glue_wwt/viewer/table_layer.py	73.46% <100%> (+0.41%)	:arrow_up:
glue_wwt/viewer/tests/test_wwt_widget.py	100% <100%> (ø)	:arrow_up:
glue_wwt/viewer/qt_data_viewer.py	94.11% <0%> (+5.88%)	:arrow_up:

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