Title location and alignment

[ivan-boikov]

Adds manual plot title positioning with a tuple (x,y) (as was done in PGFPlotsX) for GR and Plotly.
Also adds title font alignment for GR, Plotly and PGFPlotsX since it also impacts title position.

Problems:

• PGFPlotsX puts title center at given coordinates and then aligns, while GR and Plotly align entire title relative to given coordinates. I am not sure what's the expected behaviour, so I left as it was.
• UnicodePlots does not allow control over title location
• InspectDR only supports offset in vertical direction, moreover plot area is painted over title, so title cannot be placed inside plot as in GR, Plotly and PGFPlotsX

Partially fixes #4407 (since only GR and Plotly are fixed).

[codecov]

Codecov Report

Base: 80.87% // Head: 80.80% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (805b6d3) compared to base (341348a).
Patch coverage: 62.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4410      +/-   ##
==========================================
- Coverage   80.87%   80.80%   -0.07%     
==========================================
  Files          30       30              
  Lines        7439     7445       +6     
==========================================
  Hits         6016     6016              
- Misses       1423     1429       +6     

Impacted Files	Coverage Δ
src/backends/gr.jl	88.88% <60.00%> (-0.45%)	⬇️
src/backends/pgfplotsx.jl	76.26% <100.00%> (+0.04%)	⬆️

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[t-bltg]

It is difficult to test this without ref images (since it is backend dependent), and I don't think adding some is necessary, so LGTM.

Problems:

This is already an improvement over the current state: it is left for another PR.