Require Observables 0.4 & improve inference/specialization

[timholy]

This is in preparation for the official release.

[codecov-io]

Codecov Report

Merging #5 (e59ee8d) into master (1f3fb7e) will increase coverage by 2.60%.
The diff coverage is 97.70%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   90.72%   93.33%   +2.60%     
==========================================
  Files           6        6              
  Lines         981     1005      +24     
==========================================
+ Hits          890      938      +48     
+ Misses         91       67      -24     

Impacted Files	Coverage Δ
src/GtkObservables.jl	92.72% <88.88%> (-1.03%)	⬇️
src/widgets.jl	88.95% <94.11%> (+6.63%)	⬆️
src/graphics_interaction.jl	94.65% <94.44%> (+0.15%)	⬆️
src/precompile.jl	99.15% <99.09%> (-0.85%)	⬇️
src/extrawidgets.jl	94.90% <100.00%> (+1.57%)	⬆️
src/rubberband.jl	96.55% <100.00%> (+1.14%)	⬆️

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