Qt5 packages *together* manage a header file qconfig-modules.h which
`#includes` other headers. A given Qt5 package may get a line in there,
or it might not: that depends on package settings.

On installing a package, add the line (if needed), on deinstall, remove
the line (if it was there). There's a tricky case, too: upgrading
a package that *had* a line, but no longer has one. That behaves like
deinstall. Previously we used @postexec and @postunexec lines, but
these were fragile in their own special way. See the linked PRs for
examples -- or tcberner's comment that his qconfig-modules.h file
*was* a screenful, and is now much shorter (and alphabetical, too!)

An additional complication comes from needing to update qtchooser,
*if* it is installed and *if* the package needs it. There was another
set of @postexec lines for that.

Migrate all the update logic to a shell script (there already was one
for deinstall, now make it do triple-duty). From the template file,
`pkg-change.in`, we generate a pkg-install and a pkg-deinstall
script that does the right thing.

PR:		253356 253360
Reported by:	Nimaje on IRC
Reviewed by:	tcberner