xfce4-hdaps-dialogs.c: replace GTK_STOCK macros with their values.

The GTK_STOCK_* macros are all deprecated and are throwing warnings in
gtk+-3.x. But apparently, hard-coding the value of the macro is fine?
This is what the examples in libxfce4ui do at least. On the other
hand, the "stock items migration guide" suggests that we should use
newer icon names from the freedesktop.org specification.

Having tried the new names, I am happy to say that they don't work at
all, leaving me with the obviously-correct solution in this commit: to
replace the deprecated macros with their hard-coded values verbatim.