Bulk changes via RoadContainer panel

[TheDuckCow]

If we have multiple RoadPoints selected, and a user presses add or remove lane, we should (try) to perform that operation.

Some requirements:

• It would be nice to recognize if the user is holding "shift" to just "add" the selection of the next roadpoint, instead of replacing selections
• If a user clicks "add lane", it should simply add one lane on the according side to all selected roadpoints
• If a user clicks "remove lane", it should should reduce the number of lanes by up to 1, noting the conditions:
  • If there is already 0 lanes in that direction, no change
  • If there is one lane left, and there's at least one other road point in the selection which has more than one lane left, leave this road point alone
  • If all selected roadpoints have 1 or fewer lanes left, then reduce them all to 1 (thus, it becomes a one way road).
• Bonus points: Moving the road width, should width, and gutter profiles all to this panel, and removing them as export vars (with the intention that this panel eventually may replace all of the export vars).

Issue forked from: https://github.com/TheDuckCow/godot-road-generator/issues/5

[TheDuckCow]

This update idea here is that, since a "Name Road" is meant to be represented by a RoadContainer, the bulk panel could actually be the same UI as the RoadPoint panel, but situated on the RoadContainer node. We would just need to decide how it behaves (e.g. if it always auto snaps the entire road container to the same settings, and whether we'd need to expose another set of UI areas to allow for copying of all other properties, like width etc from a specific roadpoint).

[TheDuckCow]

Taking a humble start to this, there's been many times when I had wished I'd already implemented this feature.