WT-13574: Remove WTableView header left margin & padding

[m-seibt]

• Factor out WTableView::columnWidthWithPadding in order to use magic number in one place
• WTableView header cells:
  • Remove 6px margin-left and reduce padding-left in order to align with content cells
  • Adapt JS function resizeColumn and WTableView::columnWidthWithPadding
• Round WTableView::setColumnWidth argument width to pixels instead of to whole units

screenshots	without sorting	with sorting
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Resolves https://redmine.emweb.be/issues/13574

[RomainMard]

Thanks for the pull request.

We already had a fix for this issue but it is indeed better to completely remove the margin here so I have adapted it to the changes you made.

As a side note, changes to Bootstrap 5 theme should be done in the src/themes/bootstrap/5/wt folder. This is what is used to compile Bootstrap 5 for Wt. How to compile Bootstrap 5 for Wt is described in doc/development/themes.md.