Artist should be able to select target for image import (new board vs. current board)

[audionerd]

There are three ways to bring an image in:

1. "File > Import Image"
2. Drag image to window
3. Mobile image import

Right now, 1 & 2 go to a new board, 3 goes to current board.

We should allow the artist to select which method they prefer, and remember it. Possibly a dialog box on first use, and remember the setting in prefs after that? Or just a prefs setting to start?

Related to #381

[gordielachance]

I don't think this is a good solution because

• no one likes too much dialog box
• about remembering the setting: you don't always import images the same way : once you'll need to import it to current board, another time you'll need to import it as new shot.

About importing an image to the current board, see #853 (comment)

[setpixel]

Yeah I think we need two menu options per gordie's point:

Import Image(s) as new boards
Import Image into current board

[audionerd]

Implemented in #1279