On illumos/Solaris, the passwd command requires a real terminal and
won't work over pipes. This adds a guarded code path so that the "Change
Password" dialog works on these platforms. The Linux/BSD behavior is
unchanged.

Fixes #384

Expose new titlebar mouse actions in the window properties
dialog (middle-click, right-click, scroll wheel).

Note: requires the matching marco changes to work properly.

This one was quite tricky ... On systems where pam_passwdqc
is installed the prompt for password after succesfully authenticating
is not:

```
New password:
```

But:

```
Enter new password:
```

See:

https://github.com/openwall/passwdqc/blob/7bf266fcf6ce9782b73722664e36dabd68f48c48/pam_passwdqc.c#L64

passwdqc is installed by default in Gentoo, but might be also installed
in other Linux distributions.

For screen reader's sake, make the shortcut empty when hidden,
otherwise a more or less random value is presented.  The actual value
is whatever the render was last used with, remembering cell renderers
are reused for every row.

This could also be fixed in the accessibility implementation side, but
we rely on GTK's, so it's easier to clean it up here than modify GTK or
subclass it further.