If installing lxqt after the full kf5 libraries, and kf5 was not
installed in /usr (typically /opt/kf5), two packages need help
finding the needed headers. Add the appropriate environment
variables to the instructions for those packages (lxqt-panel and
screengrab).
fully in /opt.
This happens because, similar to lxqt-panel, it doesn't know how to find
the headers in /opt/kf5 and assumes they'll be found in /usr. Work
around this with CXXFLAGS.
Also updated the short description for screengrab itself
There are several packages that are not critical for the lxqt desktop
but are useful. A user can test out lxqt without the packages in apps.
It is useful to have at least one terminal emulator when testing, but
xterm will do.
