More improvements

Removed some unused code, started implementing flash progress updates, renamed the "Flash!" button to "Start". Next up: implement a page for while flashing, improve the verify flash page, allow plugins to access shared_utils directly.

flash_tool_plugins/__init__.py

@@ -31,20 +31,10 @@ def initialise_buttons(self):
         """Initialise buttons."""
         pass
 
-    @abstractmethod
-    def flash(self):
-        """Flash a device."""
-        pass
-
     @abstractmethod
     def select_partitions(self):
         """Prompt the user to select partitions to flash."""
         pass
-
-    @abstractmethod
-    def flash(self):
-        """Flash."""
-        pass
 
     def __init_subclass__(cls, **kwargs):
         """Register this plugin when it's subclassed."""