Added support for OpenRaster format.

[Banbury]

Storyboards can now be edited in Krita and other programs, that support
the OpenRaster format.

Known problems:

• It is slow. Reloading an image after editing it, takes several seconds. It would be good, if there were a way to block the GUI and display an hour glass cursor.
• No toolbar icon. I wasn't able to create an SVG, that would show up in Storyboarder. I reused the Photoshop icon as a placeholder.

I had a lot of problems with the current layer system. That's why I created those issues :).

[setpixel]

@Banbury could you make it a preference so that it replaces the photoshop button with Open Raster? Also, what should the icon look like? I can design it.

[Banbury]

@setpixel A switch in the preferences I could do. But I don't know how to switch icons in the toolbar. That's hard coded in the HTML file.

I've attached the icon, I wanted to use. But it doesn't show up. That's the Krita logo btw.

Calligra_Krita_icon_bw.zip

[rawbee]

I think the way I would like to see this implemented is something like this:

1. Add a preference for the user to be able to switch which external editor is used.
2. Add the Open Raster reading and writing capabilities to file-helper.js
3. Update the openInEditor function in main-window.js to look at the preference, and generate the correct file.
4. Update the openInEditor function in main-window.js to watch, read, and correctly update the Open Raster and their corresponding boards.

[Banbury]

@rawbee I have updated the PR as per your suggestions. Could you have a look, if that is okay now.

I have added support for radio buttons in the preferences. But someone needs to apply some styling to them. I'm no artist.

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