Botframework-composer: Electron app: Add file menu options

[bartbilliet]

Minor UX improvements for the Electron app

• Currently the buttons _New_, _Open_ and _Save As_ are only available from the Home screen.
• The _Save As_ button on the Home page is grayed out when no bot is opened (logically), and gets enabled when a bot project is loaded. With the current design of the Home page, it is not intuitive for a user to come back to Home to find the _Save As_ action after making changes to a bot project, as most of the actions on the Home page are related to the creation of a new bot (from scratch or via samples).

Suggestion:

• Add the actions _New_, _Open_ and _Save As_ to the _File_ menu in the Electron app, so they can be accessed from anywhere.
• Maybe the _Export_ actions (as .zip & as skill) can be added under the _File_ menu as well?

[hibrenda]

@sangwoohaan @mareekuh may take a look

[tdurnford]

@sangwoohaan Can you weigh in?

[sangwoohaan]

This is a great suggestion. We will take a look into it