Cleanup references to drawing and vmlDrawing. If the workbook does no…

[JanMarvin]

…t contain these directories, the references may disturb third party software (Excel usually does not bother for some reason). fixes #285

This is fix1 from #231

[codecov-commenter]

Codecov Report

Merging #301 (8959636) into master (1c0295d) will increase coverage by 0.03%. The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage   66.48%   66.51%   +0.03%     
==========================================
  Files          34       34              
  Lines        8939     8950      +11     
==========================================
+ Hits         5943     5953      +10     
- Misses       2996     2997       +1     

Impacted Files	Coverage Δ
R/WorkbookClass.R	59.23% <94.44%> (+0.14%)	:arrow_up:

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[JanMarvin]

If a comment is added to a workbook and the workbook is saved:

1. the xl/drawings/vmlDrawing1.vml is written
2. the xl/drawings/vmlDrawing1.vml is imported to wb$vml[[1]]
3. this happens without checking whats in wb$vml[[1]]

This might be dangerous and requires testing!

[JanMarvin]

needs unittests as well