Style disabled menu entries (bsc#1182668)

Problem

Enabled and disabled menu entries look identical, making impossible to distinguish them.
The pull-down menu limits are not clear, which makes it to look fusioned with other UI elements using the same background color.
https://bugzilla.suse.com/show_bug.cgi?id=1182668

Proposed solutions

To grayout disabled menu items, in a similar way it is already done in SLE installation theme
To use a border for the pull-down menu.

Tests

Tested manually by using the style sheet editor (Ctrl Shift Alt S).

Screenshots

Click to show / hide some screenshots

--- | Stylesheet | Before | After | |-|-|-| | installation.qss | ![installation.qss before](https://user-images.githubusercontent.com/1691872/115430456-4cf6b800-a1fc-11eb-8557-09648f7055dc.png) | ![installation.qss after](https://user-images.githubusercontent.com/1691872/115430523-5849e380-a1fc-11eb-8d74-d22701a927c9.png) | |cyan-black.qss | ![cyan-black.qss before](https://user-images.githubusercontent.com/1691872/115431123-de662a00-a1fc-11eb-9dce-331900a5d62c.png) | ![cyan-black.qss after](https://user-images.githubusercontent.com/1691872/115431170-e920bf00-a1fc-11eb-9f52-37c5b580f946.png) | | white-black.qss | ![white-black.qss before](https://user-images.githubusercontent.com/1691872/115431233-fb026200-a1fc-11eb-88b5-886208442959.png) | ![white-black.qss after](https://user-images.githubusercontent.com/1691872/115431345-1a00f400-a1fd-11eb-87c7-ee99b8432fd2.png) | | highcontrast.qss

_{(only separator changes)}

| ![highcontrast.qss before](https://user-images.githubusercontent.com/1691872/115430667-7a436600-a1fc-11eb-9f9a-0101ce2b0d67.png) | ![highcontrast.qss after](https://user-images.githubusercontent.com/1691872/115430940-b37bd600-a1fc-11eb-94fe-d3977823797e.png) |

openSUSE / branding