Outline intensity setting

This MR adds a tiny outline intensity combobox to Breeze settings in the Shadow tab. 
I chose shadow tab since the outline is drawn in the shadow drawcall, 
so maybe this will help people realise it's not part of the borders.

BUG: 465948
FIXED-IN: 6.0

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Here's examples with default breeze dark theme

Off

![Screenshot_20230302_181427](/uploads/22145f9d7b336f5437a4953933c4ab4d/Screenshot_20230302_181427.png)

Low

![Screenshot_20230302_181459](/uploads/807e3fb887e73a6fcd7c6cd70c713b26/Screenshot_20230302_181459.png)

Medium

![Screenshot_20230302_181437](/uploads/a14485ccbc0c2c6e7fc020972f96ab94/Screenshot_20230302_181437.png)

High

![Screenshot_20230302_181510](/uploads/30141d08ed9eb48eae4bba13c37db2fd/Screenshot_20230302_181510.png)

Maximum

![Screenshot_20230302_181452](/uploads/2eeb591d95a296adfa2daee4a985351e/Screenshot_20230302_181452.png)

Currently the effect only appears after pressing OK. This is same with all the Breeze window decoration settings afaik.

_Also, dont mind the settings window having same outline at the same time in the images: I'm not using the dev environment so it doesnt affect anything outside of the preview image. In dev env it changes everywhere._