Session Log

[vkbo]

For a while now, the length of each writing session has been logged to a file in the project meta folder. This PR adds a GUI to display this log, and the sum total time.

The dialog can be opened from the menu under Project > Session Log.

[codecov-io]

Codecov Report

Merging #59 into master will decrease coverage by 1.12%.
The diff coverage is 19.14%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
- Coverage   66.77%   65.64%   -1.13%     
==========================================
  Files          36       37       +1     
  Lines        5781     5920     +139     
==========================================
+ Hits         3860     3886      +26     
- Misses       1921     2034     +113

Impacted Files	Coverage Δ
nw/gui/timelineview.py	96.59% <ø> (ø)	⬆️
nw/project/project.py	82.44% <100%> (ø)	⬆️
nw/constants.py	100% <100%> (ø)	⬆️
nw/gui/mainmenu.py	93.76% <100%> (+0.06%)	⬆️
nw/gui/sessionlog.py	13.84% <13.84%> (ø)
nw/gui/winmain.py	68.81% <50%> (-0.02%)	⬇️

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