Show installation instructions on Cursorless install

[pokey]

The problem

It's possible that users will just install Cursorless without looking at the full installation instructions. Thus, they won't know that they need Talon, knausj, cursorless-talon, etc

The solution

We should show them the installation instructions in a webview the first time that they run cursorless

see also #1953
Should probably merge with the above

[AndreasArvidsson]

This will be implemented in #1953