Use filename for Window Menu

[markusoelhafen]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] Now with support of multiple windows, the "window" menu can be used to switch between them. But currently all windows are named Alva.

Describe the solution you'd like It would help, if the window menu uses the filename to distinguish between them. New Alva files, that don't have a name yet can use untitled, untiled 2, etc to be distinguishable. The "Welcome Screen" should also get its own window name (currently also named Alva.

Example from Sketch: