delete existing files explicitly in windows installer

Earlier, we just removed the installation directory recursively.
The uninstallation code was refactored and reused at the time of
installation to remove files explicitly mentioned, which prevents
problems when the installation directory is incorrectly chosen i.e.
fixes #1585
add script to update the chroot environments
fix compilation failures when not building with patched QT/WebKit

Apparently, no one uses the plain vanilla build as no one reported
any issue for a long time. This will get it to compile and work at
least for a sample page.