Docview Meta Data

[vkbo]

This PR adds a panel at the bottom of a viewed document with all the documents referencing it. If there are no backlinks, the panel is not visible.

In addition, all references set in a document with the @ keywords are now converted to clickable links in the document viewer.

This PR is related to issue #76.

[codecov-io]

Codecov Report

Merging #109 into master will increase coverage by 0.09%.
The diff coverage is 72.6%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   61.06%   61.15%   +0.09%     
==========================================
  Files          45       46       +1     
  Lines        6911     7028     +117     
==========================================
+ Hits         4220     4298      +78     
- Misses       2691     2730      +39

Impacted Files	Coverage Δ
nw/gui/statusbar.py	97.27% <100%> (ø)	⬆️
nw/gui/winmain.py	65.5% <100%> (+0.51%)	⬆️
nw/gui/elements/docviewer.py	69.15% <33.33%> (-9.26%)	⬇️
nw/project/index.py	85.65% <45.45%> (-3.87%)	⬇️
nw/gui/elements/viewdetails.py	73.91% <73.91%> (ø)
nw/convert/text/tohtml.py	88.6% <91.3%> (+2.64%)	⬆️

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