Improve D-Bus struct handling: as properties

[mvidner]

Responding to #97.

• properly document: unserializing a struct makes a frozen array

Before

Before this PR, the return signature for getting a property was a Variant:

org.freedesktop.DBus.Properties.Get(..., ..., out VARIANT value)

With no type hint for the Ruby -> D-Bus data conversion, a property with a nontrivial type, such as STRUCT, would get returned incorrectly:

$ busctl --user --list introspect org.ruby.service /org/ruby/MyInstance | grep -E NAME\|MyStruct
NAME                            TYPE      SIGNATURE RESULT/VALUE                            FLAGS
.MyStruct                       property  (sss)     3 s "three" s "strings" s "in a struct" emits-change # not visible here
$ dbus-send --session --print-reply --dest=org.ruby.service /org/ruby/MyInstance org.freedesktop.DBus.Properties.Get string:org.ruby.SampleInterface string:MyStruct
method return time=[...] sender=:1.985 -> destination=:1.987 serial=24 reply_serial=2
   variant       array [
         variant             string "three"
         variant             string "strings"
         variant             string "in a struct"
      ]

After

Now, Property.Get has a special case to look at the signature declared for the property itself.
So a STRUCT, ieven though mplemented at the service side as an Array, gets returned as a STRUCT.

$  dbus-send --session --print-reply --dest=org.ruby.service /org/ruby/MyInstance org.freedesktop.DBus.Properties.Get string:org.ruby.SampleInterface string:MyStruct
method return time=[...] sender=:1.990 -> destination=:1.991 serial=12 reply_serial=2
   struct {
      string "three"
      string "strings"
      string "in a struct"
   }

[coveralls]

Coverage decreased (-0.3%) to 84.389% when pulling 6addc2e on struct-properties into bf4b868 on master.

[jreidinger]

ruby dbus does not use common TODO: or FIXME: comments?