initial browser/installer commit, injector get_tab and stateful utils

- Integrated plugin downloader/installer. It accepts POST requests at /browser/install_plugin, containing an artifact (basically an author/repo string like you'd find on github), and a release version, then fetches the zip file from the repo releases and unzips it inside the plugin dir, after asking for user confirmation (pop-up message in the plugin menu).
- Injector get_tab method. Basically get_tabs with the usual search for a specific tab. Decided to implement this because it was needed again and again, and we kept pasting the same list search one-liner.
- Utilities now have access to the main PluginManager class

plugin_loader/loader.py

@@ -7,7 +7,7 @@ from os import path, listdir
 from importlib.util import spec_from_file_location, module_from_spec
 from logging import getLogger
 
-from injector import get_tabs
+from injector import get_tabs, get_tab
 
 class FileChangeHandler(FileSystemEventHandler):
     def __init__(self, loader, plugin_path) -> None:
@@ -190,6 +190,6 @@ class Loader:
         return {"plugins": self.plugins.values()}
 
     async def refresh_iframe(self):
-        tab = next((i for i in await get_tabs() if i.title == "QuickAccess"), None)
+        tab = await get_tab("QuickAccess")
         await tab.open_websocket()
         return await tab.evaluate_js("reloadIframe()")