This automatically keeps the installer's idea of Gtk version with the one
used to compile geany in sync.

Traditionally we use the bundles from gtk.org to compile geany, and this
is also used for the installer. With msys2, we can use precompiled
msys2 binaries. These exist for GTK3 as well so we can actually provide a
GTK3 based installer. The installer naturually should reflect this.
Msys2's GTK2 is also newer.

Use the new script gtk-bundle-from-msys2.sh to extract the precompiled
GTK binaries from msys2 / pacman for packing the installer (can also be
used to make a zip for sharing).

mkdir gtk; cd gtk; ../scripts/gtk-bundle-from-msys.sh [-c] [-z] [-2 | -3]

waf is also adapted to process geany.nsi.in. This implies geany.nsi is now
under _build_, not in the root anymore.