lxqt: better organize system packages

Split packages in three categories, all of them going into the system
package list:
- pre-requisite packages
- core packages
- optional packages

Add a new configuration option 'environment.lxqt.excludePackages' to
specify optional LXQt packages that should be excluded from system
packages.

Add 'gvfs' as a pre-requisite package, needed by 'pcmanfm-qt' to
handle virtual places, like "Computer" and "Network".