Have an indentation guides option

## Problem

Some times extraneous spaces get inserted at the start of lines when cutting and pasting from another source. It is then hard to tell which lines are correctly indented by a multiple of 4 spaces.

## Proposal

Vertical dotted lines at the correct tab spacing in the space preceding indented lines would be useful for detecting extraneous spaces.

## Prior Art

Other IDEs like Geany implement this
You can easily see in the example that there is an extraneous space before a closing brace.

首先检查 elementary/code 仓库中的编辑器视图及其现有的缩进或显示设置。当某个选项能够在缩进的行之前，按照配置的制表符间距显示垂直点状辅助线，并包含提案中所示的间距时，该功能即完成。

由索引模型根据 Issue 内容生成。