Improve name of exported file

[stropitek]

Currently it exports it using an id that is not human friendly, and it also doubles the extension on it. E.g. 0a6ded3b-658b-49b6-8dd8-d81d4aa54040.png.png

We can simply use the image title, making sure we remove any existing file extension on it.

[targos]

The name should be prompted to the user like on other software.

[stropitek]

The name should be prompted to the user like on other software.

The prompt could be used for other options like adding annotations or not (c.f. see #119)

I made a typo in my first message (edited since). I meant you should use the file name using the image's title.