hot reloading, plugin instantiation, plugin main method

- The Loader now watches for file changes in the plugin directory, and will (re)import when a new plugin is created, or an existing one is modified. This is implemented by means of the watchdog library
- Plugin classes are now instantiated (and therefore require a self arg in every method). This way they can maintain a state during the runtime of the loader (or until they are reloaded), and share data between methods.
- Plugins can now have a __main() method, which can include long-running code. Every plugin's main method is ran in a separate asyncio task.
- Plugin methods that start from __ are now uncallable from javascript. This can be helpful when implementing unfinished/development versions of methods.

plugin_template.py

@@ -7,8 +7,13 @@ class Plugin:
 
     tile_view_html = ""
 
-    async def method_1(**kwargs):
+    hot_reload = False
+
+    async def __main(self):
         pass
 
-    async def method_2(**kwargs):
+    async def method_1(self, **kwargs):
+        pass
+
+    async def method_2(self, **kwargs):
         pass