Fixed extraction of sheet names and identification of sheets with comments

[heseber]

The commits fix two issues:

1. There was a copy&paste bug for the identification of sheets with comments with the effect that sheets with drawings instead of sheets with comments were set to hasComment=TRUE.
2. Some xlsm files I wanted to read returned wrong sheets if sheets were requested by sheet names. It turned out that this is due to veryHidden sheets with an empty r:id. Sheet names for such sheets are now ignored.

[codecov]

Codecov Report

Merging #436 into master will increase coverage by 0.05%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
+ Coverage   60.23%   60.29%   +0.05%     
==========================================
  Files          30       30              
  Lines        7142     7150       +8     
==========================================
+ Hits         4302     4311       +9     
+ Misses       2840     2839       -1

Impacted Files	Coverage Δ
R/wrappers.R	42.7% <100%> (+0.09%)	⬆️
R/readWorkbook.R	83.33% <100%> (+0.23%)	⬆️
R/loadWorkbook.R	69.97% <80%> (+0.41%)	⬆️

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