Implement 'showSaveDialog' method of plugin API

[ashumilova]

Add ability to show the save dialog to the user, which allows to select a file for saving-purposes, with the Plugin API usage. There should be possible to provide save options:

• filters (the list of file extensions)
• caption of the save button
• default resource to be shown by the dialog
  See the docs: https://code.visualstudio.com/docs/extensionAPI/vscode-api#SaveDialogOptions

The task includes following items to be done:

• define the API for showSaveDialog
• document the API
• develop the defined API
• test it on the sample plugin

[vitaliy-guliy]

Code to test

    const showSaveDialogCommand: theia.Command = {
        id: 'plugin-api-show-save-dialog',
        label: 'Plugin API: Show Save Dialog'
    };

    disposables.push(
        theia.commands.registerCommand(showSaveDialogCommand, (...args: any[]) => {
            const options = {
                saveLabel: 'Save now',
                defaultUri: theia.Uri.parse('file:///home/vitaliy/temp'),
                filters: {
                    'Text files': ['txt', 'text'],
                    'JS, TS': ['ts', 'tsx', 'js'],
                    'HTML': ['htm', 'html']
                }
            } as theia.SaveDialogOptions;

            theia.window.showSaveDialog(options).then(value => {
                if (value) {
                    theia.window.showInformationMessage('Save result: ' + value.path);
                } else {
                    theia.window.showInformationMessage('Save dialog has been closed');
                }
            });
        })
    );