Check partial keybindings to match current context

[vinokurig]

What it does

Check partial keybindings to match current context. VsCode uses the same approach: https://github.com/microsoft/vscode/blob/1bf341e6d2b0963e6260853e570d2655f987ffcb/src/vs/platform/keybinding/common/keybindingResolver.ts#L279-L292
They look for a suitable command by checking the context before viewing partial keybinding notification in the status bar

How to test

1. Apply VsCode Vim extension
2. Disable the Vim plugin by executing F1 => Vim: Toggle Vim Mode
3. Press g key.
   See that the status bar didn't change.
   Theia at current master state shows a notification in the the status bar, that the first part of the keybinding was pressed.

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

[paul-marechal]

lgtm, the status bar seems to display the correct information.

[akosyakov]

I have not tested, but expecting only active chords make sense to me.