Bug and suggestions for Downloads page

Bug: 
Two options for Linux downloads

Suggestions

• Product dropdown should default to something more useful than JOM
• "Qt online installer" should be "Unified Qt installer" to match above language
  • Likewise, it would be less confusing if Qt Installer Framework could be written or named differently
• "Qt" should be "Qt Source Code" or something a little less ambiguous
• Add subsections to the product dropdown
  • At least separate Qt Sources from Tools
  • Add a separate subsection for legacy tools to give less priority (Qt 3D studio or Qt4 tools)
• Each product should have a description
• Each product and subproduct should have a compatibility chart
  • Linux Unified Qt Installer 4.7 does not work on Ubuntu 18 or 20, etc
• Change the dropdown flow from Product->Version->Host to Host->Product->Version
  • Detect the host OS and set the Host field to that value
    • This hides products that are incompatible, reducing clutter
• In general, always hide incompatible options unless the user clicks some "Show incompatible hosts/versions" checkbox

Goals:

• Make it less likely for clients to download an incompatible version of Qt
• Reduce chances of downloading the wrong package
• Help the client understand what they are downloading
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