EphyLocationController: remove unused properties

https://bugzilla.gnome.org/show_bug.cgi?id=666808
Merge EphyWebViewSecurityLevel and EphyLocationLockState

EphyWebViewSecurityLevel has lots of unused members. Once they're
removed, they correspond one-to-one with EphyLocationLockState. Merge
these so we don't have to convert between two identical enums.

https://bugzilla.gnome.org/show_bug.cgi?id=666808
Remove various show_lock properties

EphyTitleBox and EphyLocationEntry now know to show the lock whenever
the security level is not EPHY_SECURITY_LEVEL_NO_SECURITY.

https://bugzilla.gnome.org/show_bug.cgi?id=666808
Display warning icon if mixed content is detected

Since an attacker can modify some contents of these pages, we should
display a warning icon instead of a secure lock, following the convention
of major browsers.

https://bugzilla.gnome.org/show_bug.cgi?id=666808
Display security status in a popover

Instead of bringing up the certificate details dialog when clicking on
the lock icon in the address bar, instead bring up a popover that
displays information about the security status of this site, including a
button to open the certificate dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=666808
Open certificate popover if title box lock clicked

We show two locks in the header bar: the lock in the location entry, and
the lock in the title box. Previously, only the lock in the location
entry was actually functional, but clicking the lock in the title box
ought to work, too, since you should not need to open the location entry
to perform any actions.

Arguably the lock should be removed from the location entry so that this
functionality only exists in one place, but this patch leaves both locks
intact.

https://bugzilla.gnome.org/show_bug.cgi?id=666808