Improve the opacity animation UX with a few changes

This updates/changes a few things with opacity animation keyframes and the animation curves docker

1. Improve display for animation curves view. There are vertical lines between each frame and the node is slightly bigger to select
2. Scrubbing header area updates current playhead time on animation curve docker
3. If you right click on a frame in the timeline docker, there are new menu entries for adding and removing opacity keyframes. Called "tween" for now.
4. Moved some opacity keyframe actions to the .action file to be more consistent with other stuff
5. Creating a new opacity keyframe now uses the current frames opacity instead of resetting to 0%

Note: opacity keyframes do not work with the animation frames cache. When testing these changes, turn off OpenGL which will disable the cache. Getting this stuff to work with the frames cache will have to be another ticket (unless someone knows of an easy fix).

Fixes this bug: https://bugs.kde.org/show_bug.cgi?id=408669

Fixes/Improves a few points from this ticket https://bugs.kde.org/show_bug.cgi?id=399120

Test Plan

Make an animation and try to add and remove opacity keyframes through right-click frame menu. Make sure new opacity keyframes keep previous opacity instead of resetting it open animation curves docker and scrub through timeline header open animation curves docker and scale view to see vertical lines

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.