Update Lyo Designer required P2 plugins lists - for plugin and source installs

[berezovskyi]

eclipse_lyo/install-lyo-designer.html suggests to install "Sirius Integration with Xtext"

Do we need it? I think we can remove it from the requirements: https://oslc.github.io/developing-oslc-applications/eclipse_lyo/install-lyo-designer.html

Also, the page suggests to install those plugins even if not installing from the source - but our plugins will pull the necessary dependencies if installed from /{stable,edge}. We should clearly split the page into 4 options:

1. Installing standalone Lyo Designer (stable).
2. Installing stable as a plugin.
3. Installing edge as a plugin.
4. Running from source (edge or any other branch).