Add global settings

[o-sdn-o]

Changes

• Add global settings
  • on posix: /etc/vtm/settings.xml
  • on win32: %programdata%/vtm/settings.xml
• Remove VTM_CONFIG env variable support
• Change settings loading order

  graph LR
      subgraph Settings loading order
      direction LR
          B("Init hardcoded
          settings")
          B --> C["--config #lt;file#gt;
          specified?"]
          C -->|Yes| D["Merge #lt;file#gt;"]
          C --->|No| F["Merge global"]
          F --> G["Merge user wise"]
          D ---> H["Merge DirectVT packet
          received from parent"]
          G --> H
      end
  

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Closes #465