Allow opening multiple documents

[Feuermurmel]

At least on OS X, it does not seem to be possible to have multiple documents open at the same time. Choosing File > New or File > Open … from the menu automatically closes the current document (without asking whether to save changes made to it, see #23) and then opens the next document.

[dalboris]

Unfortunately, I am not currently planning to allow multiple documents to be opened at the same time in the same instance of VPaint. I know that this is a common and convenient feature in many applications, but it is more rarely seen in animation packages, primarily because the interface of this type of programs is already complicated enough without.

I may change my mind on this (especially if it appears that a lot of people are using VPaint as a 2d editor, without using the animation features) but my current opinion on the subject is that VPaint would be better without the feature than with (i.e., I prefer not bloating the UI with this feature, even if it causes the inconvenience to open different instances of VPaint to work on different projects).

That being said, indeed, doing "open" shouldn't cause the loss of your current document. Instead, it should ask you whether you want to open it in a new instance of VPaint or replace the current one, and whether you want to save your current document if it contains unsaved changes. I'll try to implement this behaviour soon.