* Plugin callsigns, filechangehandler thread bug fix, plugin file perms

- Plugins are now assigned a callsign (a random string), which they use for all internal identification, like resource fetching and method calls. This is to ensure that plugins only access their own resources and methods.
- Made FileChangeHandler send off events to a queue, that is then consumed by the Loader, instead of calling import_plugin on its own, since that caused weird issues with the event loop and the thread watchdog is using.
- Plugins are now owned by root and have read-only permissions. This is handled automatically.

* Improved general look and feel of plugin tab

* Make all plugin entries have the same padding between them

* Make "No plugins installed" text look the same as "No new notifications"

Co-authored-by: WerWolv <werwolv98@gmail.com>

- 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