Signal manager: Add a tooltip about mismatching types

[janezd]

I drafted something and I know it's ugly:

• It assumes that __dynamicEnabled being false is always a consequence of type mismatch. (True for now, but not necessarily in the future.)
• It doesn't update the tooltip if the user renames the source or sink widget after sending the data. (Minor glitch, but still...)

The latter would require connecting some distant signals, the former requires checking the reason for mismatch (i.e. comparing the signal types).

@ales-erjavec: is this a useful start or would you approach it differently?

@ajdapretnar: is the message OK?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.49%. Comparing base (e9297c5) to head (76f3e74).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #324      +/-   ##
==========================================
+ Coverage   76.47%   76.49%   +0.01%     
==========================================
  Files         102      102              
  Lines       21248    21257       +9     
==========================================
+ Hits        16249    16260      +11     
+ Misses       4999     4997       -2     

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[ajdapretnar]

I think the message is ok.

[ales-erjavec]

@ales-erjavec: is this a useful start or would you approach it differently?

It's fine.

[janezd]

• Now the tooltip changes if the user edits widget titles.
• It is still assumed that disabled dynamic link is due to invalid signal type. I won't dug into this.
• Existing tests needed to be fixed because the anchor now needs a widget (to get its title). I also test the tooltip.

@ales-erjavec, please review and merge if it's OK.