Change paste to allow copying files/folders from file manager, set DB defaults

[samuel-w]

Possibly fixes #758. Also changed default behaviour of add_source_to_table, fixed paste button.

Edit: The test is because paste caused a crash due to the new source tab behaviour.

[codecov-io]

Codecov Report

Merging #759 (e32c861) into master (a876aed) will decrease coverage by 2.31%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #759      +/-   ##
==========================================
- Coverage   73.45%   71.14%   -2.32%     
==========================================
  Files          53       53              
  Lines        3583     3587       +4     
==========================================
- Hits         2632     2552      -80     
- Misses        951     1035      +84     

Impacted Files	Coverage Δ
src/vorta/views/source_tab.py	78.34% <87.50%> (+6.44%)	⬆️
src/vorta/models.py	80.50% <100.00%> (-0.50%)	⬇️
src/vorta/keyring/darwin.py	0.00% <0.00%> (-92.11%)	⬇️
src/vorta/autostart.py	63.63% <0.00%> (-33.34%)	⬇️
src/vorta/network_status/darwin.py	33.89% <0.00%> (-27.12%)	⬇️
src/vorta/notifications.py	71.21% <0.00%> (-13.64%)	⬇️
src/vorta/keyring/abc.py	80.00% <0.00%> (-8.58%)	⬇️
src/vorta/utils.py	62.50% <0.00%> (-6.00%)	⬇️
src/vorta/network_status/abc.py	82.35% <0.00%> (-5.89%)	⬇️
src/vorta/config.py	94.44% <0.00%> (-5.56%)	⬇️
... and 3 more

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[m3nu]

Looks good, but can't test it.

[Hofer-Julian]

Works fine on GNOME with nautilus