text: add support for invoke_action request

Add a request to be called by the client when the word currently being
composed is tapped by the user. Input methods often use this information
to offer more word suggestions to the user.

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

protocol/input-method.xml

@@ -127,6 +127,10 @@
       <arg name="hint" type="uint"/>
       <arg name="purpose" type="uint"/>
     </event>
+    <event name="invoke_action">
+      <arg name="button" type="uint"/>
+      <arg name="index" type="uint"/>
+    </event>
   </interface>
 
   <interface name="input_method" version="1">

protocol/text.xml

@@ -135,6 +135,10 @@
       <arg name="hint" type="uint"/>
       <arg name="purpose" type="uint"/>
     </request>
+    <request name="invoke_action">
+      <arg name="button" type="uint"/>
+      <arg name="index" type="uint"/>
+    </request>
     <event name="commit_string">
       <description summary="commit">
         Notify when text should be inserted into the editor widget. The text