Fix a bug in the "Change Selection" action, whereby changing your
selection and picking files from another directory results in:
"Dangerzone does not support adding documents from multiple
locations. The newly added documents were ignored."
To fix this, change the output directory when we change selection as
well.
Fully test the update check logic, by introducing several Qt tests.
Also, improve the `UpdaterThread.get_letest_info()` method, that gets
the latest version and changelog from GitHub, with several checks.
These checks are also tested in our newly added tests.
Now that sample_doc was renamed to sample_pdf it could cause some
confusion the fact that that the TestBase class had an attribute called
sample_doc which referenced the sample PDF.
By removing this attribute and passing the fixture instead we are
following a more pytest-native approach of passing arguments explicitly.
