Change UI to scenes rather than code generated

[TokisanGames]

It's very difficult for us to improve and build upon the UI when generated by code. It should be built as a scene in Godot and instanced by the code.

eg

const UI = "res://addons/gterrain/ui/GTerrainUI.tscn"

ui = load(UI).instantiate()
ui.ted = self
add_control_to_bottom_panel(ui, "Terrain Editor")
make_bottom_panel_item_visible(ui)

[outobugi]

I strongly disagree. I find it much easier to work on the UI when it's all in code.
The UI code could be separated to multiple scripts instead of sub classes.