CheckBox: support styling indeterminate state of tri-state check boxes

[DevCharly]

This PR enables full styling indeterminate state of tri-state check boxes (issue #919).
See check box icons in red rectangles (left is FlatLightLaf, right is FlatIntelliJLaf):

There are a lot of new UI properties to do this. See source code of FlatCheckBoxIcon in this PR.

Samples

Change blue border to gray:

CheckBox.icon.indeterminateBorderColor = $CheckBox.icon.borderColor

Also change blue indeterminate checkmark to gray:

CheckBox.icon.indeterminateBorderColor = $CheckBox.icon.borderColor
CheckBox.icon.indeterminateCheckmarkColor = $CheckBox.icon.disabledCheckmarkColor

Use white background for FlatIntelliJLaf:

CheckBox.icon[filled].indeterminateBackground = $CheckBox.icon.background
CheckBox.icon[filled].focusedIndeterminateBackground = $CheckBox.icon.background
CheckBox.icon[filled].indeterminateCheckmarkColor = $CheckBox.icon.checkmarkColor

Theme Editor

Above screenshots are made with FlatLaf Theme Editor.