Qt5 and Qt6 compatibility

[luca-della-vedova]

Bug fix

Fixed bug

Closes #155

Fix applied

Follows the approach proposed in #155 (comment), we can:

• Remove Qt dependencies and just use the Qt version that is brought in by gz-gui.
• Use unversioned functions for CMake.
• Check at the qml level for the Qt version and adapt the class names with a leading underscore for qt >= 6.
• Remove LightTuning altogether since most of its content is now available in upstream Gazebo, it has also been untested for a long time since it's not part of the default plugins but I can try to readd it if there is need for it.

I tested this in a 24.04 distrobox with either rolling or jazzy and it seems to work OK, but happy to get another pair of eyes.
Note that rolling has a few other issues, open-rmf/rmf_visualization#87 and gazebo failing to run (at least for me) because of an invalid command. I had to run Gazebo manually instead of using the launch file:

$: export GZ_SIM_RESOURCE_PATH=~/rmf_ws/install/rmf_demos_assets/share/rmf_demos_assets/models:~/rmf_ws/install/rmf_demos_maps/share/rmf_demos_maps/maps/hotel/models/
$: gz sim -r -v 3 /usr/local/google/home/lucadv/noble_ws/install/rmf_demos_maps/share/rmf_demos_maps/maps/hotel/hotel.world -z 100

GenAI Use

We follow OSRA's policy on GenAI tools

• I used a GenAI tool in this PR.
• I did not use GenAI

[arjo129]

This is a very elegant solution to the problem of maintaining two versions of the same UI. I tested it and it seems to be working.