add :unindent property

[nativerv]

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Description

Adds a property to control label unindenting/stripping leading spaces (default is true). This partly addresses #647 (although it is about using raw strings as widgets, but with this you can use label to work around it).

Usage

Example of a custom raw-label widget that does not strip leading spaces:

(defwidget raw-label [text ?class ?truncate-left ?show-truncated]
  (label
    :unindent false
    :text "${text}"
    :class {class}
    :truncate-left {truncate-left ?: false}
    :show-truncated {show-truncated ?: true}
  )
)

Showcase

Screenshot of my bar:

The rightmost elements use the above raw-label widget to display left-padded strings that i get from my scripts correctly, so the bar does not flick around on updates.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing