spin box: Improve for M3

[peter-ha]

Resolves #398

normal:

hovered:

pressed:

Comments:

• size of up/down buttons: normal push button size (i.e. height of 40 dp)
• emphasis: As mentioned in the ticket emphasis is like the very low button emphasis
• spin box height has been chosen to be the same as the menu height (48 dp). Different
  controls have different heights here, e.g. outlined text field has 56 dp and chips
  only 32 dp. So we could also chose another height here.
• size of icons: Same as push button icons.
  (Panel size is 40 dp, icon padding is 11 dp, so icon size is 40-2*11=18 dp)

[uwerat]

I adjusted the code to make use of QCursor::pos() - instead of storing the position as skin hint

[peter-ha]

Commits for reference: 932d34a and b7c2b27