Consider exposing task stopped state

While testing the cyclic tasks for https://github.com/hardbyte/python-can/pull/412 I noticed we don't expose whether a task is currently active or stopped. Specifically I was trying to see the state of `ThreadBasedCyclicSendTask` which has a `stopped` boolean attribute which isn't in the Task api and ASFAIK isn't thread safe.

If this was modified to be a property we could check an underlying driver/kernel for various backends to query if the task is active.

先定位 Task API 和 ThreadBasedCyclicSendTask，接著檢視 pull request 412 中關於 cyclic-task 的工作。確認現有的 stopped 屬性如何公開，以及如何跨後端查詢工作狀態。完成的條件是，公開的工作 API 能夠回報工作是作用中還是已停止，而不依賴不安全的直接屬性。

由索引模型根據 Issue 內容生成。