writerfilter: Make the generated ::attributeAction's readable.

Change-Id: I8461c374841ebd98d5767469ab2d931a2854921e
writerfilter: Use if's in ::attributeAction's instead of switches.

This only improves readability, otherwise the code size is exactly the same.

[And there is only one place where we are checking for 2 values - case
NN_wml|DEFINE_CT_FtnEdn: in OOXMLFactory_wml.cxx.]

Change-Id: Id6dc6b0cdd0e42f2215e978e1d4d91472969bbb8