feat: Add DBus desktop notification support.

[iphydf]

This is nicer than the Qt built-in tray notifications. It only works on Linux. On other systems, tray notifications are pretty ok.

Fixes #208.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 127 lines in your changes missing coverage. Please review.

Project coverage is 11.02%. Comparing base (2d6bb67) to head (61e8315).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
...tform/desktop_notifications/desktopnotify_dbus.cpp	0.00%	108 Missing ⚠️
...c/platform/desktop_notifications/desktopnotify.cpp	0.00%	14 Missing ⚠️
src/widget/widget.cpp	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   11.04%   11.02%   -0.02%     
==========================================
  Files         294      295       +1     
  Lines       21226    21369     +143     
==========================================
+ Hits         2344     2356      +12     
- Misses      18882    19013     +131     

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