FavoriteStore external change support

Bug Description

- Favorite store should react to external entities changing the favorites key and emit the correct added/removed/reordered signals on that change

- Will need to "diff" the current list and the new list

- Tests should include cases where the entire list has changed, or something has been added as well as the list being re-ordered (in additon to normal testing of individual changes)

Hi, I'm trying to fix this bug as a bytesize (although it isn't tagged as one).

Although my changes are little (see linked branch), I ran into this problem: once an icon is removed, it cannot be recreated. Any hints?

This bug was fixed in the package unity - 5.2.0-0ubuntu1

---------------
unity (5.2.0-0ubuntu1) precise; urgency=low

* New upstream release.
- Unity needs a way to switch (tab) between windows on current workspace
(LP: #863399)
- compiz crashed with SIGSEGV in BamfLauncherIcon::NameForWindow()
(LP: #865840)
- Gradual degradation in desktop performance. (LP: #888039)
- compiz (unity) crashes with SIGSEGV when a window is minimized.
(LP: #918329)
- FavoriteStore external change support (LP: #681503)
- Launcher - Make Launcher left of screen reveal more responsive and less
prone to false positives (LP: #765819)
- Window auto-maximise functionality should be disabled on monitors with a
resolution above 1024 x 600 (LP: #797808)
- Dash: very high latency responding to input (LP: #828582)
- Dash - Behaviour of the 'All' button in the Dash filters broken in
several ways (LP: #841864)
- alt-tab - The app title in the top left of the top bar should change as
the alt-tab focus changes (LP: #855516)
- Keyboard shortcut - Add keyboard shortcut hint overlay that is displayed
when a user presses and holds the Super key (LP: #855532)
- Unity crashes when started in an environment without utouch support
(LP: #860707)
- Dash - Remove Dash Home shortcut icons (LP: #885738)
- Dash - Most Frequently Used apps change to Recently Used, without
Launcher favorites (LP: #893214)
- Should have a launcher on every monitor (LP: #915944)
- Launcher autohide behaviour on multi-monitor (LP: #915946)
- the unity wrapper should kill compiz before restarting it (LP: #919132)
- Launcher - Implement workspace/launcher cross interactions (LP: #690143)
- Application icons should only display windows from the current workspace
in the window spread (LP: #689733)
- Notification area ("system tray") missing when using dual monitors of
different sizes, with their bottoms aligned (LP: #778256)
- Clicking Nautilus launcher icon fails to open a Nautilus file explorer
window when copying a file and all other Nautilus windows are closed /
bamf should skip the taskbar (LP: #784804)
- Dash - the search box is not aligned correctly relative to the Launcher
BFB button (LP: #838904)
- Dash - A expand/collapse arrow is missing from all the filter category
headers (LP: #841870)
- Dash - the filter buttons should not have a mouse over state
(LP: #838901)
- Dash - the "Filter results" text is the wrong size, wrong font weight,
and aligned incorrectly in both the vertical and horizontal axis
(LP: #863240)
- Add SUPER+TAB switching mode that enables the user to switch
applications via the Launcher (LP: #891620)
- Software Centre - automatically add app icon to launcher (LP: #761851)
- Compiz add transparency to titlebar along with the panel (LP: #912682)
- The search box is too opaque and dark (LP: #913717)
- Dash - Make statefulness of Dash Home and Dash Lenses consistent
(LP: #914759)
- Unity 5.0: "All" button for filters render as "..." (LP: #91...