While the class_init() defaults readonly property to FALSE, it's not marked
to be constructed, thus this value doesn't propagate to the corresponding
variable. That would not be a problem, but the instance's init() function
set the readonly to TRUE. Let's make it the same.

Closes #19

Closes #22

The problem was that EWebDAVDiscover (used to find out CalDAV calendars/memo
lists/task lists and CardDAV books) wanted to receive a proxy ESource, for
which it requires either ESourceRegistry or ESourceRegistryServer. As
the EWebDAVDiscover API didn't have any such argument it always created its
own ESourceRegistry instance, which had been dropped shortly afterwards. This
drop also meant that the internal GDBus object scheduled its own events on
the ESourceRegistry's thread main_context (to unsubscribe D-Bus signals), but
this main_context didn't have enough time to flush the queue, which resulted
in a leak of the main_context, because the events in it referenced it (similar
to circular reference). The fix was to flush the queue before closing
the thread and to reorganize the ESourceRegistry's dispose() function, thus
the flush is done on filled main_context.

While I've been in it, some structures unnecessarily long held a GCancellable
object, which also means a file descriptor, thus I fixed that as well.

As the last, but not least, I added a new API for the EWebDAVDiscover to
be able to provide also a function to reference an ESource, thus there is
no need to create the ESourceRegistry instance, thus it is quicker and more
effective now.

Reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1600489

Related to #14

At least the EBookCache can store non-UTF-8 values into the database
within the INSERT statement, which require conversion to UTF-8,
otherwise an "[Invalid UTF-8]" error is shown on the client side instead.

Correct issues in the previous commit.

Closes #14

Closes #14

Closes #13

Closes #12

The folders->pdata can be NULL when folders->len is 0, thus do not
claim the runtime warning when there had been passed no folders.

It could happen that one thread had been working with the connection
while another thread disconnected it, which could lead to a race
condition on the connection object, especially when the code execution
had been within that object.

The change makes sure to either add a reference on the connection
object while working with it or it adds a connection lock which is
acquired for the whole time when the connection object is used.

Closes #4

Based on patch by Watson Yuuma Sato.

Closes #2

The evolution-source-registry console could contain a runtime warning:
WARNING **: secret_service_search_sync: must specify at least one attribute to match
which was due to calling secret_service_search_sync() with an empty
hash table of attributes. Adding an 'eds-origin' attribute to the keys
and search for it helps to make the module do what it was supposed to do.