Re-add Touchpad applet in safe read-only form

I removed the applet entirely in !455 (merged) to fix multiple bug reports related to people bricking their laptops by accidentally disabling their touchpads without a mouse handy, leaving them with no way to re-enable touchpad input.

However in retrospect, removing the applet entirely was too heavy-handed, as it can serve a valid purpose as a status notifier to remind people who have manually disabled their touchpads that they have done so. The problem was always the fact that you can use it to disable the touchpad.

The two commits of this MR re-add the applet, and then make it into a simple reas-only status notifier that shows you when the touchpad has been disabled, but does not let you do so yourself. This makes the applet safe enough to keep in upstream code.

This is deliberately done in two commits so that people or distros that absolutely hate the applet being read-only can revert 294507de to get the applet back into its original unsafe yet more feature-full state.

I'd like to get this into Plasma 5.23.4 if possible. No new translations are added compared to the state of the applet in Plasma 5.22.