fix(a380x/mfd): Change soft key behaviour to page & add individual line scroll through KCCU in FPLN

[BravoMike99]

Fixes #[issue_no]

Summary of Changes

Changes softkey navigation in F-PLN page to scroll through "pages".
Disables down navigation softkey once the end of the flightplan is in view.
Adds individual line scroll navigation through KCCU (old softkey behavior).

Screenshots (if necessary)

2025-01-29.00-39-04.mp4

References

https://youtu.be/AI8yFhuEa4Q?t=5201

Additional context

Discord username (if different from GitHub):
bruno_pt99

Testing instructions

1. Program a flight plan of your choice and go to the F-PLN page on the MFD.
2. Using the arrow up and arrow down keys on the KCCU verify that the FPLN page scrolls line by line.
   2.1 Once the start of the flightplan is visible, the up arrow up key does nothing.
   2.2 Once the end of the flightplan is displayed (if you have an alternate, it will be the end of it instead), the arrow down key does nothing. Verify that the down soft key is disabled/greyed out on the bottom right of the MFD.
3. By using the up or down soft keys, verify that it is possible to navigate through "pages". This means, when going "down" the last waypoint of the current list will be the first one of the next list. When going up, the opposite applies, the first waypoint of the "current" list becomes the last after pressing the up soft key. The exception to this is when the start or end of flightplan (includes alternate if present) are reached, at which point the page should display the start or end of the flightplan data. Feel free to mix this one with 2).
4. Perform a modification in your flight plan such that a temporary is created and scroll down the page to a list of waypoints which did not exist before/only exist on temporary. After doing so erase the temporary flightplan and the page should update to fit the existing waypoints like the example video.

How to download the PR for QA

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