Fix bug in Linux keyboard input sender

Held modifiers weren't being released again.

src/keyboard_input/input_sender_X11.cpp

@@ -120,6 +120,7 @@ void sendTokensAsInput( const std::vector<Token> tokens )
         if ( isModifier( token ) )
         {
             sendKeyPress( token, KeyStatus::Down, display );
+            heldInputs.push_back( token );
             continue;
         }