Formatting (on macOS) and mining hashrate displayed in Mining Tab that was added in PR969 but testing on the GA PR967 build

[seanPhill]

While formatting issues seemed to be addressed in PR969 at some point, this remains after the merge to master.
[Testing multiple OSes found this only to affect my macOS systems.]

1. The font size and alignment in the number of threads selection box only shows the top of the digit(s).
2. The button next to it, which increments that number has an odd font or symbol, which should be adjusted to look more like a mathematical plus sign (with equal width and height) than a Roman cross (taller than wide).
   (I don't really understand the use of an increment button without a matching decrement button, or a "Use Max Threads" button without a matching "Use Min Threads" button. That would balance things nicely, if done. (Not essential.)

The mining hashrate did not adjust for a long time, and does not get erased after stopping mining, at least not soon enough that I ever saw it. (I suppose this might be an artefact of code elsewhere that is not specific to this mining tab.)

Likewise, it is possible to select ProgPow, and start mining, and have the hashrate show up, selecting any number of threads, while not using any external mining software, and happening to only have a pair of 3GB GPUs on a Mac Pro. Is it really attempting to mine? "Mining at 352247 H/s". Even if so, I'm not sure selecting more threads than GPUs would make sense. (I haven't ever really tested ProgPow mining before, as I'm running macOS and GPUs with less RAM than the DAG size. Surely it's not simply drawing on the 32 GB of main RAM in the computer?)

Veil Qt is from the GA build of merge 967, which occurred after the 969 merge, ... but not sufficiently 'after' for me to be sure any final changes in 969 and 966 might be been included. I'll check the 969 merge separately now! ...
... Checked the 969 merge and the Mining Tab formatting still has the same problems.

Checked on macOS Big Sur (late-2013 Mac Pro Xeon 6-core) and macOS Catalina (late-2013 iMac i7 4-core),
Windows 10 (old dual core Intel MacBook Pro) !! The formatting is okay in Windows, and the decrement button is visible.
Ubuntu 20.04.3 LTS (old dual core Intel MacBook Pro) The formatting is also okay in Ubuntu.

Something exclusive to macOS format or fonts must be doing this.