FormLayout: Set mnemonic only if buddy is visible and doesn't have one
Reduces the number of dead or redundant mnemonics.
For example, with a CheckBox as buddy, the CheckBox will already have an mnemonic on its own, no need to have one on the form label next to it, too. Also, for example when the control is replaced by a static label conditionally (e.g. the "single refresh rate" in KScreen), the mnemonic makes no sense if the buddy control is not visible.
Before:
Refresh rate: 60 Hz (i)
High Dynamic Range: [ ] Enable HDR
After:
Refresh rate: 60 Hz (i)
High Dynamic Range: [ ] Enable HDR
(Note that KScreen doesn’t have mnemonics on most controls right now but a fix for that is coming up next)