feat(appearance): support gap < active_hint

[adil192]

Lets the user set gaps less than the active window hint.

I recognize that this was an intentional UX decision aimed to prevent the active window hint overlapping with other windows. So I've left this behavior unchanged except that I've made it optional now.

Behavior before:

• Increasing the active_hint also increases the gap if lower.
• The user cannot decrease the gap below the active hint.

Behavior now:

• Increasing the active_hint also increases the gap if lower.
  This is unchanged and keeps the sensible default of gaps >= active_hint.
• The user can now decrease the gap if they wish.

Closes #1683

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