Upgrade Qt to 6.2 LTS

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Upgrade Qt to 6.2 LTS

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This is done. The develop branch is now on Qt 6.2.4 LTS. Changes will be shipped with v0.11.0.

Building Qt from source on Ubuntu 16.04 was too painful. Looking at Bloom's download analytics, I can see that literally no one has selected 16.04 when downloading Bloom, so I've decided to move the building to an 18.04 VM. This means Bloom's distributed binary will no longer work on Ubuntu 16.04 (or Debian 9 ("Stretch")-based systems), from version 0.11.0. This made things a little easier for me, but still quite difficult, as Qt's binaries only support 20.04 and later. So I still had to build Qt from source, but with fewer dependencies to worry about.

As part of this work, I've also done the following:

• Moved resources out of src/resources.xml and into the CMakeLists.txt files, using the qt_add_resource() CMake macro
• Separated Insight resources from the other application resources. Insight resources are now defined in src/Inisght/CMakeLists.txt
• Moved away from using DT_RPATH to specify dependency paths in Bloom's binary. We're now using DT_RUNPATH. I was sure I had tried this before, and it didn't work, but I can't remember why. It's working at the moment, but I may have to revert back if I find any issues.