Extension Manager: add missing security token

initialize remote data in one go

Previously each extension was fetched separately from the API, this
fetches all installed ones in one go, speeding up the first open on cold
cache significantly.

Extension Manager: (automatic) code style fixes

Extension Manager: added the last missing GUI tabs

Extension Manager: check repo access in handle()

Still need to check if the behaviour on missing internet works okay.

Extension Manager: Template GUI

template tab works now, fixed installed status

Extension Manager: add installer to GUI

Now the GUI uses the new Installer class. More work has been done on
the GUI itself

Extension Manager: overhauled list display style

The GUI is still not fully functional again.