This drops the self-compiled 'grep' version which caused issues
with recursive searches on Windows as well as with non-ASCII
characters in paths.
The pre-compiled 'grep' binary from MSYS2 (matching the target ABI)
seems to solve probably all of the known problems on Windows.
The only drawback is that we need to ship a few additional MSYS2
libraries on which the 'grep' binary depends.

Closes #1229 and #1260.

The files in share/glib-2.0/schemas are necessary for GTK3,
so don't delete them but everything else on the same directory level.

Update the list of dependencies, include sort.exe and grep.exe,
create a information file with all download links of included
binaries and re-structure the script for better readability.
See #560 for details.

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.