Supporting widgets in slices for loc and iloc

[hoxbro]

Part of the solution for https://github.com/holoviz/hvplot/issues/771

Checklist:

• [x] Add support for slice in resolve_dependent_value.
• [x] Support for pn.widgets in slice for transform.params().
• [x] Add loc to df_dim.
  • [ ] I'm not sure if it should be moved up into dim.
• [x] Solve widget values in loc and iloc.
• [x] Support integer lookup for pandas in iloc.

[codecov-commenter]

Codecov Report

Merging #5352 (e6d5c3d) into master (5ce1e1d) will increase coverage by 0.01%. The diff coverage is 93.40%.

@@            Coverage Diff             @@
##           master    #5352      +/-   ##
==========================================
+ Coverage   88.07%   88.09%   +0.01%     
==========================================
  Files         301      301              
  Lines       61957    62032      +75     
==========================================
+ Hits        54567    54644      +77     
+ Misses       7390     7388       -2     

Impacted Files	Coverage Δ
holoviews/util/transform.py	86.91% <84.21%> (+0.91%)	:arrow_up:
holoviews/core/util.py	86.05% <100.00%> (+0.02%)	:arrow_up:
holoviews/tests/core/test_apply.py	100.00% <100.00%> (ø)

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