gtk2: Add a button to show/hide the passphrase.

* gtk+-2/pinentry-gtk-2.c (HIG_TINY): New.
 (confirm_unhiding): New.
 (show_hide_button_toggled): New.
 (create_show_hide_button): New.
 (create_window): Add a show/hide button.

This is an alternative implementation to the checkbox
reverted with rev. 71b51e0.
The patch is based on a patch by Werner Koch <wk@gnupg.org>
modifications done by aheinecke are:

• Use of strings provided by the gpg-agent
• Switching the visibility state of the edit.
• Using a monospace font for the label to avoid size changes when toggling the button.
• Use of a default button label for cancel in the confirm dialog as the agent only sends a string that is useful for show.