Add compatibility with libgcrypt 1.6.

Closes #129
Parse the libgcrypt version in the cmake module.
Use the libgcrypt SALSA20 cipher if available.
Add PasswordEdit class.

It displays passwords in monospace which closes #51
Add ability to load icons with on/off state.
Use an icon for the button that shows/masks passwords.

Closes #38

Additionally make use of the new PasswordEdit class where possible.
Add an option to show passwords by default.

Closes #93
Merge remote-tracking branch: upstream/master