Position of GadflyDisplay in Base.Multimedia.displays

[Mattriks]

• I've built the docs and confirmed these changes don't cause new errors
• I've tested this in the REPL, IJulia and vscode. I presume it also works in Juno.

This PR

• fixes Gadfly doesn't show plots in julia-vscode plot pane. #1311

Example (in IJulia)

# The GadflyDisplay doesn't need to override IJulia's Display 
using Gadfly
Base.Multimedia.displays

3-element Array{AbstractDisplay,1}:
TextDisplay(Base.TTY(Base.Libc.WindowsRawSocket(0x0000000000000298) open, 0 bytes waiting))
 Gadfly.GadflyDisplay()                                                                     
 IJulia.InlineDisplay()

[codecov-io]

Codecov Report

Merging #1373 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1373   +/-   ##
=======================================
  Coverage   90.55%   90.55%           
=======================================
  Files          38       38           
  Lines        3961     3961           
=======================================
  Hits         3587     3587           
  Misses        374      374

Impacted Files	Coverage Δ
src/Gadfly.jl	78.14% <100%> (ø)	⬆️

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[bjarthur]

@montyvesselinov can you please confirm this PR does not break support for terminalextension that i added at your request. i don't think it will but want to double check.

[bjarthur]

@montyvesselinov this works for you?