password keyring is positively hazardous

[124655b5d049 sigaint org]

It is quite easy to accidently erase a password irrevocably when copying
it.  This makes the app unacceptable, as it leads either to keeping
handwritten backup copies or to loss of valuable passwords.

To improve the situation:

Add an EXPORT context menu item to the password keyring menu.  It
should bring up an output file selection dialog.  The file should be
created using gpg --armor --symmetric, using the current passphrase of the
keyring as the symmetric key.  Decrypting the file yields a human readable
list of names and passwords.  Options on the file selection dialog could
be provided to specify other output formats and encryption options.

Add support for importing a password keyring to the FILE IMPORT menu item.

Change the password details dialog:

*COPY PASSWORD button copies old password field to clipboard

*HIDE PASSWORDS checkbox when unselected displays the contents of password
fields

*CURRENT PASSWORD field contains the existing password

*NEW PASSWORD field for entry of a changed password

*VERIFY field which must match the NEW PASSWORD

*SAVE button is enabled when NEW == VERIFY and NEW not null.  It changes
the password in the keyring and dismisses the dialog.

*CANCEL button dismisses the dialog without any changes to the keyring.