Allow for spell icons to be shown above/below/on the threshold lines.

Open questions:

• How do we handle overlapping icons? (e.g. two abilities both with the same resource cost).

• • Threshold lines currently overlap with priority order of: Usable (green) > Under Resource (white) > Unusable (red).

• Zoom in on the icons at all to get rid of the default "rounding" border?

• Positioning options

• • "Centered on" -- Above, Below, Middle
• • X/Y offset

• Size options for icon

• Icon border size

• Implement for; Add "Show icon" checkbox for each threshold line for each spec; Spec-wide enable/disable

• • Demon Hunter
• • • Havoc
• • Druid
• • • Balance
• • • Feral
• • Hunter
• • • Beast Mastery
• • • Marksmanship
• • • Survival
• • Priest
• • • Holy
• • • Shadow
• • Rogue
• • • Assassination
• • Shaman
• • • Elemental
• • Warrior
• • • Arms
• • • Fury

First release of this will be with 9.1.5.0-release (has been in 9.1.0.17-beta01 onward). There will need to be a second pass to add some more QoL features, possibly including but not limited to:

• Staggering icons from thresholds with same value (or are overlapping)
• Color shading of icons to match line colors
• Cooldown timers and/or swirl

Need to add Chimaera Shot for Marksmanship to it's own line/icon.