Preferred Applications only listed apps that were registered in
mimeinfo.cache. Apps installed in unusual ways, or with a stale or
missing cache, wouldn't show up as options for web browser, mail reader,
and other defaults.

This change now scans each installed .desktop file directly for its
MimeType entry, so apps missing from the cache are still found.

Also fixes a longstanding bug where duplicate messenger entries weren't
actually being filtered out.

Fixes #150

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.