As Timeseries: Fix crash on sparse data

[janezd]

Issue

Fixes #106.

Description of changes

• Problem when the user selected an attribute was caused by a check whether the column contains any nans.

  I cannot replicate the bug when using a sequence. Maybe this was already fixed somewhere. @ajdapretnar, please check.

• The second commit updates the widget's code to use

  • context settings, with variables instead of strings
  • QGridLayout, for more accurate spacing
  • deferred commit
  • WidgetPreview

Includes

• [X] Code changes
• [X] Tests

[codecov-commenter]

Codecov Report

Merging #210 (d917c20) into master (238da17) will increase coverage by 0.34%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   69.81%   70.16%   +0.34%     
==========================================
  Files           7        7              
  Lines         858      858              
  Branches      163      163              
==========================================
+ Hits          599      602       +3     
+ Misses        210      209       -1     
+ Partials       49       47       -2     

Impacted Files	Coverage Δ
orangecontrib/timeseries/timeseries.py	87.64% <100.00%> (+1.68%)	:arrow_up:

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