Migrate all logical pixel Property values to Length.

[xStrom]

This is a goal of the plan outlined in #1264, that properties should use Length for their logical pixel lengths. A bunch of properties use Length already and with this PR here all of them will.

Specifically, this PR changes the following:

Files	Changed fields
masonry_core/src/properties/types/gradient.rs	RadialGradientShape::FixedCircle
masonry_core/src/properties/border_width.rs	BorderWidth::width
masonry_core/src/properties/box_shadow.rs	BoxShadow::blur_radius
masonry_core/src/properties/corner_radius.rs	CornerRadius::radius
masonry_core/src/properties/padding.rs	Padding::left/right/top/bottom
masonry/src/properties/slider.rs	TrackThickness / ThumbRadius

Additionally, all the constructors and call-sites were updated as well.

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Draft because it depends on #1787 and currently includes all of those commits as well.