[server] Adjust TreeModule structure

This commit prepares a slighly more powerful TreeModule structure: Now,
TreeModules can finish their execution, which should make some kinds of menus
and transitions possible.

src/Forest/TreeModule.hs

@@ -8,16 +8,16 @@ import qualified Data.Text   as T
 import           Forest.Node
 
 class TreeModule a where
-  edit :: a -> Path -> T.Text -> IO ()
-  edit _ _ _ = pure ()
+  edit :: a -> Path -> T.Text -> IO Bool
+  edit _ _ _ = pure True
 
-  delete :: a -> Path -> IO ()
-  delete _ _ = pure ()
+  delete :: a -> Path -> IO Bool
+  delete _ _ = pure True
 
-  reply :: a -> Path -> T.Text -> IO ()
-  reply _ _ _ = pure ()
+  reply :: a -> Path -> T.Text -> IO Bool
+  reply _ _ _ = pure True
 
-  act :: a -> Path -> IO ()
-  act _ _ = pure ()
+  act :: a -> Path -> IO Bool
+  act _ _ = pure True
 
 type ModuleConstructor a = (Node -> IO ()) -> (a -> IO ()) -> IO ()