#1174 More consistent palette import behavior

[davidlamhauge]

This PR addresses the inconsistent behavior on opening, importing, saving and exporting palettes.
As we agreed, this is how it goes:

1. Opening a palette, replaces the existing palette with a new one. If at least one color is in use on a vector layer, a warning is given. Only one warning.
2. Importing a palette, appends the new palette to the old one.
3. Saving a palette can be done to the Pencil2D xml format.
4. Exporting a palette can be done to GPL, XML, and other formats as soon as the formats are implemented.
   I hope I've covered all (or most) issues we've talked about.
   This closes Inconsistent palette import behavior #1174

[MrStevns]

I think the export UI should be simplified, the functionality itself looks fine.

Let's keep it at only one export palette action and let the user decide which extension they want to use.

[davidlamhauge]

I have merged the newest Master branch into this branch, and fixed the typs.
I also removed the option to choose whether to export as Pencil2D or GIMP format. It works, but on my Ubuntu you must add suffix yourself.
I'm too tired to fix it today... 😪

[davidlamhauge]

Now, there is always an extension.

[davidlamhauge]

It again saves without file extension in Linux (Ubuntu 18.04).
Otherwise it works fine @candyface

[MrStevns]

The save without extension on linux should be fixed via #1299

The issue you mention stems from gtk3 not respecting its filter selector, so if no extension is given, then the file will be saved with no extension.

[MrStevns]

I'd say this is ready to be reviewed again whenever the remaining conflicts has been fixed.

[davidlamhauge]

Conflicts has been fixed.

[MrStevns]

This macro should be removed, It's redundant since it is defined above.

[davidlamhauge]

Removed

[MrStevns]

LGTM, thanks @davidlamhauge 👍