text: Add support for pre-edit string

Add support of preedit-string to the example editor client. Also add a
preedit_string request to the input_method_context interface and use
that in the example weston keyboard to first create a pre-edit string
when entering keys and commit it on space.

Signed-off-by: Jan Arne Petersen <jpetersen@openismus.com>

protocol/input-method.xml

@@ -42,6 +42,13 @@
       <arg name="text" type="string"/>
       <arg name="index" type="uint"/>
     </request>
+    <request name="preedit_string">
+      <description summary="pre-edit string">
+        Send the pre-edit string text to the applications text model.
+      </description>
+      <arg name="text" type="string"/>
+      <arg name="index" type="uint"/>
+    </request>
     <event name="surrounding_text">
       <description summary="surrounding text event">
         The plain surrounding text around the input position. Cursor is the