Use modern dialog API via support lib to show pinentry

The dialog method we were using was using a deprecated API, now it's
using DialogFragment (from the support lib). The styles changes are to
ensure that pinentry appears on top of everything using this fragment method
like we expect.
put ABS in externa/ and use the same support lib version as it
fix method name change caused by rolling back the support lib
add error message to pinentry dialog
document how to use a custom GNUPGHOME
fix the key import dialog

It wasn't getting results from the file picker, so I copied over the
original source from OpenPGP Keychain, and wired the result handling
into the main activity.
add kdevelop project files to gitignore