[Development] qt 4/5 co-installs

[Jonathan Riddell]

Looking at Qt 5 for packaging in Ubuntu.

Qt 4 and 5 contain quite a lot of files with the same names, at the
Ubuntu Developer Summit in April we discussed the issue of bin/moc
qmake etc but I see all the libraries have the same name too so
probably development environments just can't be co-installed from
distro packages.

One other thing I noticed is the $QT_PLUGIN_PATH which is used at
runtime which is used by both Qt 4 and 5.  It's set by the startkde
script to point to KDE's plugin directories so running a Qt 5
application under KDE 4 gives lots of "Old plugin format found in lib"
warnings.  I think this environment variable should be renamed to
$QT5_PLUGIN_PATH

Jonathan