EXT-332: Updated the visual style of the Buy L$ floater based on

Erica's design. Now any errors during the L$ purchase process will
give you a nice prompt to visit the SL website.

Replaced all user-visible references to the illegal term "currency"
with the term "L$".

Also fixed an outstanding bug with this floater: if you clicked the
Buy button but then cancelled the confirmation dialog, then the Buy
button would remain disabled. Now the Buy button is only disabled
during periods when you actually cannot buy.

Added initial support for local currencies, with the use of a new
[LOCALAMOUNT] field in the XUI files. When the XML-RPC server can
actually return local currency details, we can hook this up by just
changing llcurrencyuimanager.cpp.

Discussed XML-RPC interaction with Morpheus. Thanks to Erica for
the images and first cut at the XUI.