[windows] Major step in replacing gtk2 by gtk3

- Download gtk3 instead of gtk2
- Copy gtk files in ekiga directory instead of relying on a separate
  installer, since this is the current approach
- Download bundle instead of each library/header, it is easier to keep
  track of gtk libraries; note that only required libraries will
  however be put in the final .exe
- Do not download xml library anymore, since it is found in gtk bundle

Ekiga compiles well now, but the build gives some errors during final
.exe generation.  These errors (and others) will be fixed in
subsequent commits.