gui/focusable: add send_key()

gui/abstracts/focusable.py

 from abc import ABC, abstractmethod
 
+
 class Focusable(ABC):
     def __init__(self):
         self.focus = False
@@ -7,3 +8,7 @@ class Focusable(ABC):
     @abstractmethod
     def toggle_focus(self):
         self.focus = not self.focus
+
+    @abstractmethod
+    def send_key(self, key):
+        pass