Implement change divider function and button

[TheDuckCow]

As a part of this macro-scopic issue, we want to be able to move the lane divider left or right. Eventually this will be in the 3D view, but for now lets do it in our new panel.

Requirements for this task:

• Create two buttons in the new roadpoint editor panel: Move left and move right
  • Do not worry about the button layout too much, we will improve over time; this is mostly a stepping stone to the 3D widget anyways.
  • In the meantime, expect to create a row which just reads Divider: [Move left] [Move right]
• On pressing move left/right, it shifts the current position of the lane switch once in the according direction.
• The according move left or move right button should be grayed out/disabled if it's all the way to one side.
  • Moving left/right should only affect the lane direction values ultimately. It should also trigger the auto update lanes, if auto generate is on (if it is off, then leave the textures alone, as it's up to the user to define their custom lanes).
• Automatically trigger regenerating of the according road segments (the one before and after)

Out of scope:

• Creating a 3D editor widget, interactive control inside the 3D view (that will be a task after this one)
• Affecting more than one roadpoint at a time
  • We could think about an additional control or tick box which allows for "global changes", so that we can decide between the function affecting all roadpoiints vs just this single one. Or modifier keys
  • That will have more logic and scenarios to consider. Most likely we will want to sync before jumping into this. We can make it its own task
  • We could also implement it such that it tries to operate the same function on all currently select roadpoints.

Long term reference mockups

Again, per scope above, we're not making a 3D widget right now, and we're only affecting one roadpoint at a time. However this end still is still helpful as a reference:

Then using the controls we have with this task, we'd be able to accomplish the below (again, one road point at a time):

[TheDuckCow]

I'm closing this task as, we ultimately decided this would conflict with a more important center control for selecting and moving RoadPoints with ease: https://github.com/TheDuckCow/godot-road-generator/issues/49

While this change divider position would have been interesting and cool, it's easily accomplished with a few clicks and likely isn't that useful to end users anyways.

FYI @bdog2112