[core] don't let disabled bindings shadow enabled ones

[svenefftinge]

bindings contributed from VS Code extensions do currently always shadow
bindings default bindings, no matter they are enabled or not.

What it does

This change checks whether a binding is enabled before including it in any shadowing logic.

How to test

Change the binding of a (usually inactive) keybinding to Alt+Enter. (or install the python vscode extension)
No open text editor search, search something in an editor and press
Alt+Enter.

It should mark all findings setting multiple cursors.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[kittaakos]

I have tried it with the VS Code Python extension, extracted it and copied to the plugins folder. I can see the extension has started:

...
root INFO PluginTheiaDirectoryHandler: accepting plugin with path /Users/akos.kitta/git/theia/plugins/ms-python.python
root INFO Resolved "ms-python.python" to a VS Code extension "python@2020.1.58038" with engines: { vscode: '^1.38.0' }
...
root INFO Deploying backend plugin "python@2020.1.58038" from "/Users/akos.kitta/git/theia/plugins/ms-python.python/extension/out/client/extension"
...
root INFO [0e244ea0-37a0-4f8d-ae97-ebc69c53a7be][ms-python.python]: Loaded contributions.
...
root INFO [0e244ea0-37a0-4f8d-ae97-ebc69c53a7be][ms-python.python]: Started plugin.
....

but I cannot set the cursor on the search matches in editor (Cmd+F > search > Alt+Enter).

[svenefftinge]

@kittaakos sorry, I forgot to push part of my change. The PR is now updated and I have verified it once more.

[kittaakos]

I have verified it again, and it works now. 👍 Thank you for the fix!