Add next/prev keyframe shortcuts

[manongjohn]

Requested on Google forum, this PR adds shortcuts for keyframe navigation controls, Next Key and Prev Key.

The behavior of the shortcut keys changes based on the panel/window you are focused on, coinciding with how the keyframe navigation controls works on that panel:

When focused on...

• Scene/Combo Viewers - Moves between keyframes on the current level in Xsheet/Timeline
• Function Editor - Moves between keyframes in the Function Editor
• Level/Studio Palettes - Moves between color keyframes in either Xsheet/Timeline or Level Strip.
• Fx Settings - Moves between Fx keyframes

All other panels without a keyframe navigation control will use the Scene/Combo Viewer logic.

[beeheemooth]

@manongjohn Thank you very much for this feature, and for listening to user requests !

[gab3d]

Sorry to ask, but will there be default shortcuts assigned to them?
It would be nice.

What do other people think?

[manongjohn]

I was using CTRL+, (prev) and CTRL+. (Next). Basically the Prev/Next Drawing shortcuts with CTRL.

What do other software use, if they have similar keys?

[gab3d]

All main applications use a variant of the Previous/Next Frame shortcuts.
It seems good to me. 👍

[RodneyBaker]

LGTM
Very useful addition.
Thanks @manongjohn