Add support for different XML file extensions

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When you save a file with the extension .xml you can then open it with Text app from the file manager. However when you save the file as .svg or .xsl it doesn't give that option, even though they're also XML files.

However when you open a .svg file directly from the Text app it does actually use the XML syntax highlighting, but a .xsl file doesn't have any highlighting.

I don't know if there are any other file extensions that also contain XML, but I think .svg and .xsl are the most common.

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Found 2 more extensions that should be supported: .rss and .atom.

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One more thing:

.xht and .xhtm files are supported by the app (you can open them from the file manager), however they don't use the HTML syntax highlighting.