Render a Dialog when a new file is opened and current file is modified

[CriPstian]

This is in regard with #23
When the new file button is pressed, and the current file is not clean it will prompt the user with a save/don't save/cancel dialog.

• Upon pressing the save button, the user it requested to specify a path for saving the file.
• Pressing the Don't save button will create the new file and lose reference to current one
• Pressing the Cancel button will close the dialog and do not create a new file

[amitmerchant1990]

@CriPstian I tested the functionality that you've implemented but I think there's an issue in it. When you make some changes into the existing file and try to create a new file, it's giving the prompt that you've implemented. But instead of saving the existing file, it gives me the dialog to save it once again. And if I cancel that dialog the new file window is opening up, instead of giving me the original state of my previous file.

This is not the intended behaviour. Apart from this, the functionality looks fine. Can you please look into the issue I mentioned above and fix it?

[CriPstian]

@amitmerchant1990 Happy New Year!
When you have the time, I updated my PR so you can have a look.
Thanks.