Add interface definition

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>

org.pwmt.zathura.synctex.xml

+<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN" "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
+<node>
+  <interface name='org.pwmt.zathura.synctex'>
+    <method name='View'>
+      <arg type='s' name='position' direction='in' />
+      <arg type='b' name='return' direction='out' />
+    </method>
+    <property type='s' name='filename' access='read' />
+  </interface>
+</node>