login/signup window: validate input and improve UI

[ChristianSch]

• [ ] The "Login" button as well as the "Signup" button should replace (animated via slide to left) the current view. Only the feedback should be a sheet.
• [ ] atm the contents are not validated and might result in app crashes, we need to check for them before we do anything.
  • check for email validity (^*.@*.\.(*.)$)
  • check for password existence

[ChristianSch]

http://stackoverflow.com/questions/10056528/shake-window-when-user-enter-wrong-password-from-code

[ChristianSch]

http://stackoverflow.com/questions/21384514/cocoa-unified-login-window http://stackoverflow.com/questions/11971866/rounded-corner-gradient-background-of-window-in-cocoa

[ChristianSch]

This window should also be shown on startup if the user could not be logged in.

[ChristianSch]

At one point we should validate the email address for format and mark the text field red.