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/injector.py

@@ -79,21 +79,25 @@ async def get_tabs():
         else:
             raise Exception("/json did not return 200. {}".format(await res.text()))
 
-async def inject_to_tab(tab_name, js):
+async def get_tab(tab_name):
     tabs = await get_tabs()
     tab = next((i for i in tabs if i.title == tab_name), None)
     if not tab:
-        raise ValueError("Tab {} not found in running tabs".format(tab_name))
+        raise ValueError("Tab {} not found".format(tab_name))
+    return tab    
+
+async def inject_to_tab(tab_name, js):
+    tab = await get_tab(tab_name)
     logger.debug(f"Injected JavaScript Result: {await tab.evaluate_js(js)}")
 
 async def tab_has_element(tab_name, element_name):
-    tabs = await get_tabs()
-    tab = next((i for i in tabs if i.title == tab_name), None)
-    if not tab:
+    try:
+        tab = await get_tab(tab_name)
+    except ValueError:
         return False
     res = await tab.evaluate_js(f"document.getElementById('{element_name}') != null")
     
     if not "result" in res or not "result" in res["result"] or not "value" in res["result"]["result"]:
-        return False;
+        return False
 
     return res["result"]["result"]["value"]