Use variables everywhere instead of hardcoded directory names

src/gpg4win.mk.in (stamps/stamp-directories): Replace "playground" with
$(root).
(playground/install/$(1)-x86.wixlib): Rename to $(idir)/$(1)-x86.wixlib.
(playground/install/$(1).wixlib): Rename to $(idir)/$(1).wixlib.
(installers/GnuPG-VS-Desktop-$(VSD_VERSION)-$(1).msi,
installers/GnuPG-VS-Desktop-$(VSD_VERSION)-$(1)-x86.msi): Replace
"playground/install" with $(idir).

(clean-gpg4win): Replace "playground" with $(root).

• GnuPG-bug-id: T5592