Get rid of libxml dependency

[alecpl]

I don't see anything in the code that would require libxml extension. We only need DOM. Do I miss something?

[alecpl]

I see utf8_decode(), but preceded with function_exists() check, so composer.json should not require libxml.

[goetas]

I think that here the intention was to require ext-dom. Probably is a bug. (even if having ext-dom implies installing libxml...

I will be happy to change it. Can you reproduce it somehow on travis ci? removing libxml, but keeping extdom?

[alecpl]

No, I didn't try with Travis, that came from one of Roundcube users.

[alecpl]

Actually, the ctype ticket came from the user. This one is from me just reviewing dependencies.