This avoids a lot of mismatches between nsAString and char16_t*, thus removing
many getter_Copies() and ToNewUnicode() and get() calls, and generally making
things simpler.
Note: the patch removes GetDefaultPrinterNameFromGlobalPrinters() by simply
inlining it at its two callsites, which is easy with the changed types.
nsXPIDLStrings are marked as VOIDED upon initialization. Most of these local
nsXPIDLString variables are immediately set via getter_Copies(), which will
either assign a string value (using Adopt()) or do SetIsVoid(). These can be
trivially converted to nsString, which will get the same treatment.
The patch suitably converts the remaining nsXPIDLString local variable as well.
