Introduce XLinkVT mode

[o-sdn-o]

Changes

• Show the close app button if the taskbar is wide enough.
• Add environment variables support.
  Now it is possible to specify additional environment variables in the menu item configuration:

  <config>
    <menu>
      <item ... env="var1=val1" env="var2=val2" ... />
    </menu>
  </config>

• Implement XLinkVT (Cross-linked VT) mode to support OpenSSH keyboard-interactivity over DirectVT stream.
  Now it is possible to run dtvt-applications over SSH with any kind of interactive authentication that requires controlling terminal. For example, you can install vtm and the latest OpenSSH server on Windows8 and connect to it via SSH from any remote terminal (fixed in v0.9.28 - connection from unix terminal has some unfixed yet keyboard issues). You will get working all win32 console stuff directly over SSH - mouse, keyboard modifiers, and truecolor support. Just type vtm ssh user@host vtm in your remote terminal (or vtm -xlvt ssh user@host vtm).
  Also you can configure menu items for such connections:

  <config>
    <menu>
      <item ... type=xlvt cmd="ssh user@host vtm" ... />
    </menu>
  </config>

Examples of XLinkVT mode usage:

• SSHing into a Windows8 host from a Windows10 terminal and run vtm inside Win8 to get true color support and mouse working over SSH:

  vtm ssh user@win8 vtm
  

  or (-xlvt is auto enabled if the first command line argument starts with ssh)

  vtm -xlvt ssh user@win8 vtm
  

output1.mp4

• Launching remote vtm (Win8) from local vtm (Win10). In the second part of the video - launching a standalone instance of Far Manager via SSH:

  vtm ssh user@win8 vtm -r term far
  

output23.mp4

Related to microsoft/terminal#15630
Related to PowerShell/Win32-OpenSSH#1990
Related to #401
Closes #464