Tristate Switch

[christophe-thiery-fw]

QCheckBox have a tristate property, allowing a third state called "partially checked" that can be used for various purposes, e.g. an "unspecified" value, or a parent item in a tree view where children do not all have the same boolean value.
If we see Switch as a modern version of a checkbox, it would make sense to also make them support tristate.

Add a boolean property "tristate" to Switch, false by default.
When tristate is true, change Switch to the following:

• Switch in state "checked": no change: show a white circle on the right, with a background in primary color
• Switch in state "unchecked": no change: show a white circle on the left, with a background in the default background color
• Switch in state "partically checked": show a smaller white circle in the middle, with a background in the default background color (same background as "unchecked")

[oclero]

https://gitlab.com/fw-oss/qlementine/-/merge_requests