Add support for groups of keys to SignEncryptWidget

There are not yet any groups in the model, but once there are
groups all keys in the selected groups are added to the list of

GnuPG-bug-id: 5175, 5241
1 job for work/kloecker/group-support in 4 minutes (queued for 15 minutes and 32 seconds)
Status Job ID Name Coverage
failed #87409


Name Stage Failure
build_clazy_clang_tidy Build

-- Configuring incomplete, errors occurred!
See also "/builds/pim/kleopatra/build/CMakeFiles/CMakeOutput.log".
Uploading artifacts for failed job
Uploading artifacts...
WARNING: JUnitTestResults.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1