Source formatters: deduplicate and share a common MIME type list

The text/x-csrc MIME type was two positions lower in
CustomScriptPlugin::predefinedStyle()'s version of the list, but this
shouldn't make a difference because:
* text/x-chdr corresponds to the same language as text/x-csrc - C;
* text/x-c++hdr is a subclass of text/x-chdr, which is still above it.

Sharing one common list instance is faster than constructing a new list
each time it is needed.