Add widgets example backend

[skoppe]

• implemented widgets example to showcase how to use ipywidgets
• extended ci.sh to test and build it
• fix basic example readme
• made IoPubMessageSender @safe

[skoppe]

only ldc 1.16 errors out with

std/meta.d(793,37): Error: template instance F!(layout) cannot use local layout as parameter to non-global template typeOf(alias T)

will try to fix it.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 50.2%. The diff coverage is 81.84%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage     1.7%   51.91%   +50.2%     
==========================================
  Files           5        8       +3     
  Lines         234      549     +315     
==========================================
+ Hits            4      285     +281     
- Misses        230      264      +34

Impacted Files	Coverage Δ
source/jupyter/wire/kernel.d	1.96% <ø> (ø)	:arrow_up:
example/widgets/source/models.d	65.21% <65.21%> (ø)
example/widgets/source/widgets.d	81.35% <81.35%> (ø)
example/widgets/source/repl.d	85.64% <85.64%> (ø)
source/jupyter/wire/log.d	0% <0%> (ø)	:arrow_up:
source/jupyter/wire/message.d	32.43% <0%> (+32.43%)	:arrow_up:

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