Extend exportdialogs for subkeys and flags

With latest GPGME this allows users to export
an OpenSSH key for authentication with Kleopatra.
2 jobs for master in 14 minutes and 1 second
Status Job ID Name Coverage
  Build
passed #57628
build_clazy_clang_tidy

00:14:01

passed #57627
linux kf5-qt5 qt5.12

00:09:14

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