Add applicable check to @show_special

[lgoettgens]

Follow-up to #1583 by @joschmitt.

This contains the changes from #1594 to avoid conflicts. The changes of this PR are solely contained in the last commit.

This change enables us to use the @show_special macro with both signatures (with/without MIME type) extensively without the need to make sure that both dispatches exist for all show_* functions. The macro now only does something if the custom show function matches the signature of the macro.

(I cannot choose @joschmitt as a reviewer, but please have a look as well.)

[joschmitt]

Hm, I understand that at the moment it is a lot easier (and probably even necessary) to have this because not all show variants support the MIME"text/plain" stuff.
But in the long run, I think I would prefer to get an error if the method is missing. The printing system is already hard enough to debug with all the macros and io-keywords and so on.

[codecov]

Codecov Report

Attention: 51 lines in your changes are missing coverage. Please review.

Comparison is base (6fda78f) 87.17% compared to head (e4c8f1f) 85.10%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/PrettyPrinting.jl	17.74%	51 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1595      +/-   ##
==========================================
- Coverage   87.17%   85.10%   -2.07%     
==========================================
  Files         115      115              
  Lines       29565    29560       -5     
==========================================
- Hits        25774    25158     -616     
- Misses       3791     4402     +611     

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[lgoettgens]

You are right. I should rather change the way I call the macro instead of changing the macro