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This is an initial, static implementation of plugins.  The
CallsApplication has a plugin name which can be changed with a new
--provider command line option.  This plugin name is used to
instantiate the appropriate plugin when the application is activated.
From then on, the plugin cannot change.

In future, we can expand this support to include loading multiple
plugins at once, configurable through some UI.  This will have
far-reaching implications though, and complicate things like
enumerating the provider hierarchy.  There is also no practical
benefit right now; the mm and ofono plugins can't be used at the same
time because ModemManager and oFono don't play nice together, and the
whole raison d'être of the dummy plugin is undermined if you can make
use of one of the others.  So for now, we just implement one static
plugin.

Modems being added or removed were not working.  To fix this, we pay
attention to the "object-removed" event and not just
"interface-removed".

Also, to deal with ModemManager appearing and vanishing, we add a
GDBus watch on ModemManager's D-Bus object.

Finally, we provide appropriate UI feedback when it's not possible to
make a call.

Closes #15
Closes #16

Not even sure how these crept into the repository.

The return value of this callback, used for the call selector flow
box, should have a reference added to it.  The callback's return value
is annotated as "transfer full".

In the current order, the holder is finalized before we can get the
display, causing an error.  We just have to switch the two around

The display and selector item have floating references when created,
not normal references.  Therefore we need to sink both floating
references.

Closes #20

Closes #19

Closes #18

We only need to update visibility when the number of calls changes.

This allows us to get rid of the signals on the MainWindow and
simplifies things dramatically.

The main window has no need to store origins now so this simplifies
things dramatically.

Closes #3.

When call history is implemented, this should be modified to only
display the dialer if the history is empty.

We're only looking for modems with voice capabilities. Warning that
there are no modems available at all might be confusing to the person
trying to track down a problem.

This makes the calls history the page visible by default, matching the
final design.

Drop the in-window content stack witcher and replace it by a header bar
containing both a CallsHistoryHeaderBar and a CallsNewCallHeaderBar.

These will be used by the header bar to switch between the call history
page and the new call page.

This will be needed by the next commit to display an "About" dialog via
the corresponding entry in the app menu that will be added.

This also adds the PACKAGE_URL, PACKAGE_VERSION and VCS_TAG
configuration data.

These will be used in the next commit to notify of added and removed,
which will be needed to display the calls in a secondary window.

This will be used to factorize project-related information like APP_ID.