PluginSupport: Add more Signals with documentation

[t-oster]

This is a follow up of #4676

I added a few signals I need for my own plugins and added also some documentation in the signal class as comments for the constant signal-strings.

If you consider merging, please let me know, I am willing to add documentation for all other existing signal calls in order to make plugin development easier for future developers.

[t-oster]

Well I still suggest you document the signals with constants like in this PR.
However I managed to work around all the missing singals with a plugin like https://github.com/clonemeagain/attachment_preview which basically captures the whole HTML before sending to the browser and then provides an interface for plugins to mess with it. This way I can hijack any HTML element via plugin.

Thus I close this PR.
However If you feel like adding more Signal documentation or even include a DOM-Manipulation Signal, feel free to contact me.