Disabled buttons corner cases

[pkozlowski-opensource]

While investigating button-related issues I could spot some corner cases around the disabled state handling:

• it is possible to enable an individual button even if the entire group is disabled
• switching group from disabled => enabled re-enables all buttons, even the ones that are explicitly disabled.

We should probably re-think disabled state propagation from a group to individual buttons (keep both buttons and group state and calculate final states based on the 2).

[alex321]

@pkozlowski-opensource Thanks #1141, I think I've got this in a few lines. Just need to write 1 or 2 tests and I'll create a PR (probably now but if not then tomorrow).