Add an option to copy contents from a file to another

[redsigma]

The behavior should work as follow

Let's say you have an extra called Hide Library News
If the user chooses to install this extra then this extra will copy the contents from a file (mentioned in the manifest file) to another file (mentioned again in the manifest file).

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The syntax in the manifest could be like this:

Let's take for example the last line which is
"Big Picture Button + Green Notification Icon";"bigpic_n_greenicon";"big_picture_button,greenicon";"Navigation Bar"

If we add this new option then this line would look like

"Big Picture Button + Green Notification Icon";"bigpic_n_greenicon";"big_picture_button,greenicon";"Navigation Bar";"file_to_copy_from.txt";"resource/webkit.css"

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Now the line for the actual extra would actually be this

"Hide Library News";"hide_library_news";"";"Client Windows";"file_to_copy_from.txt";"resource/webkit.css"
And this will copy the contents from file_to_copy_from.txt and append them to the resource/webkit.css file

PS: maybe in the future a rework for the manifest file could be made so it uses yaml syntax