[FIRE-36987] Add configurable colours for agents loading

[TlNKER]

This fixes my ticket on jira: https://jira.firestormviewer.org/browse/FIRE-36987

Summary

Adds two new preferences so the colour of the avatar loading/bakefail "cloud" particle effect can be customized, instead of it being a fixed color. Exposed as two color pickers under Preferences > Firestorm > Extras.

Details

• New persisted settings 'FSCloudColorStart' / 'FSCloudColorEnd' store the start and end colors of the cloud particle effect.
• 'LLVOAvatar::applyCloudColor()' applies both to 'sCloud's particle start/end colors, forcing alpha to the stock 0.1 / 0.9 envelope so the effect's shape, burst, scale and timing are unchanged - only the hue is user-configurable.
• Changes to either setting are picked up live via a 'gSavedSettings' signal connection - so no restart required, and takes effect on the next avatar that shows as a cloud.
• Two 'color_swatch' controls (Start / End) added to Preferences > Firestorm > Extras, bound directly to the new settings.

Testing

• Built and installed to a local staging install with the default skin.
• Verified in Preferences > Firestorm > Extras that both pickers open the color chooser and update the setting.
• Verified an avatar currently rendering as a cloud (loading / bake fail) picks up a color change immediately, with no restart needed.

[TlNKER]

Whoops! clear oversight from my end, since I run pink default. Should be fixed now, so both Preferences float shows Red as default selected as well as the actual clouds are the usual red tones defined in cloud.xml until and if the user changes that.

[Ansariel]

Why do we need this extra function? new_value is never used, so you can just bind directly to LLVOAvatar::applyCloudColor() instead.

[Ansariel]

Should probably store the signal connection and disconnect before if connected, especially since there is a menu option to reload the particle cloud, so you will end up calling onCloudColorChanged multiple times.