Define custom parameters and make margin consistent

[philippjfr]

• Defines custom parameter types for Align, Aspect and Margin.
• Consistently set margins on components

[codecov]

Codecov Report

Merging #4528 (0caf515) into main (31b5b0b) will decrease coverage by 9.86%.
The diff coverage is 81.13%.

@@            Coverage Diff             @@
##             main    #4528      +/-   ##
==========================================
- Coverage   83.40%   73.54%   -9.86%     
==========================================
  Files         246      247       +1     
  Lines       36270    36309      +39     
==========================================
- Hits        30251    26705    -3546     
- Misses       6019     9604    +3585     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	73.54% <81.13%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
panel/layout/base.py	92.66% <ø> (-0.06%)	⬇️
panel/layout/card.py	93.65% <ø> (-0.10%)	⬇️
panel/layout/flex.py	95.65% <ø> (-0.19%)	⬇️
panel/layout/gridstack.py	52.23% <ø> (-33.06%)	⬇️
panel/layout/spacer.py	76.92% <ø> (-0.86%)	⬇️
panel/pane/markup.py	93.78% <ø> (-0.04%)	⬇️
panel/pane/vega.py	82.16% <ø> (-0.10%)	⬇️
panel/template/base.py	77.69% <ø> (-0.05%)	⬇️
panel/widgets/file_selector.py	84.70% <ø> (-0.09%)	⬇️
panel/_param.py	78.72% <78.72%> (ø)
... and 2 more

... and 42 files with indirect coverage changes

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