Prevent multiple oauth windows

[jonathontoon]

Expected Behavior

If an OAuth window has already been presented it should be the only possible instance and remain open until closed.

Current Behavior

I can continue to click the Sign in with Twitter and it keeps opening Twitter OAuth windows.

Possible Solution

Check the OAuthManager has an active window object before creating a new BrowserWindow.

Context

Could possible cause some issues with signing up, but it doesn't stop me from using Tweet Tray, just looks really jank.

Your Environment

Found on develop.

Software	Version(s)
Tweet Tray	1.0.2
Windows 10	1709