Add notification manager API

[olexii4]

Add notification manager API to the new plugin system.
Discussion about architecture and principles can be found here: #1482
This API allows implementation of notifications to show an information message. Optionally provide an array of items which will be presented as clickable buttons and can be shown using the showInformationMessage,
showWarningMessage and showErrorMessage functions.

Simple example that show an information message:

theia.window.showInformationMessage('Information message');

Simple example that show an information message with buttons:

theia.window.showInformationMessage('Information message', 'Btn1', 'Btn2').then(result => {
    console.log("Click button", result);
});