Add application menu on Windows

[fluiddot]

Related to 7084-gh-Automattic/dotcom-forge.

Proposed Changes

• Separate menu creation from setupMenu function to a new function getAppMenu. This function will be used when setting up the application menu on macOS and when displaying the app menu as a context menu on Windows.
• Create popupMenu to display the app menu as a context menu.
• Add button to WindowsTitlebar component. This button will trigger the context menu.

App	Open app menu

Testing Instructions

Windows

• Open the app.
• Observe that in the title bar there's a button on the top-left corner.
• Click on the menu button.
• Observe a context menu is displayed with the same items as the application menu on macOS.
• Click on some if the items to check the menu works as expected.

macOS

• Open the app.
• Observe the application menu is displayed as in previous versions.
• Observe no menu button is displayed in the title bar.
• Click on some if the items to check the menu works as expected.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[derekblank]

LGTM. I observed from the testing instructions that the application menu is added and available on Windows, with no change to the macOS behavior.

[wojtekn]

@fluiddot thanks, menu still works correctly on macOS and works fine on Windows. It's a significant gain as now Windows users can:

• Open About Studio
• Trigger Check for Updates
• Open 'Add Site' and Settings forms

I noticed some minor glitches:

• 'Studio -> Services' and 'Studio -> Hide Studio' do nothing
• 'File -> Close Window' exists from the app instead of closing the window
• accelerators (keyboard shortcuts) don't work
• The Edit menu looks like not needed on Windows - those actions can target form text fields, but when any field is visible, the menu can't be accessed.

However, we can tackle those under separate issues/PRs.

[fluiddot]

I noticed some minor glitches:

• 'Studio -> Services' and 'Studio -> Hide Studio' do nothing
• 'File -> Close Window' exists from the app instead of closing the window
• accelerators (keyboard shortcuts) don't work
• The Edit menu looks like not needed on Windows - those actions can target form text fields, but when any field is visible, the menu can't be accessed.

However, we can tackle those under separate issues/PRs.

Good catch @wojtekn. I'll tweak the menu items based on the platform, in the case of Windows some of them won't be included.

[fluiddot]

I've updated the app menu item for Windows in bc373f1.

The following items/menus have been removed:

• Studio/Services: This only applies to macOS.
• Studio/Hide: This only applies to macOS.
• File/Close Window: Closes the app instead of the current window.
• Edit: Seems only relevant for macOS:
• Window: Seems only relevant for macOS:

[wojtekn]

Window: Seems only relevant for macOS:

Actually, Window -> Minimize worked as expected on Windows. The Window -> Zoom did nothing.

[fluiddot]

Actually, Window -> Minimize worked as expected on Windows. The Window -> Zoom did nothing.

Yep, I also noticed that when testing. We could keep the Windows menu with only the Minimize item, but I found it redundant as we have the minimize button in the title bar.