Provide a new papirus-folders-add tool

[SmartFinn]

Since we got a lot of requests to add another folder colors, I understood that we can't cover all them. As to me, the better solution is to add a tool that allows users to add their own colors to the folders and keep only core colors built-in the icon theme.

Currently, I see the tool as:

• a separated script for creating the custom color folders
• an option to call GUI color picker with Zenity
• wiki page(s) for sharing commands between users for creating the custom color folders

Next I see two paths, each of them has its proc and cons.

The first path is to be able to user changing colors for only special folder-custom icons.

Proc:

1. Easy to implement.
2. No need to manage list of colors.
3. After the icon theme upgrade, need to generate limited quantity of icons.

Cons:

1. Unable to create two and more custom colors and switching between them with papirus-folders only. It requires running the papirus-folders-add script each time before switching.

The second path is to allow the user to add unlimited color folders.

Proc:

1. Easy switch between two and more pre-generated color folders.

Cons:

1. Requires to keep all colors added by a user in a file (.csv) and add option to add/remove items from the file.
2. After the icon theme upgrade, it needs to generate ~409*N, where N is a number of user's color folders.
3. It requires a post-remove script for papirus-icon-theme to correctly remove all added icons.