terminal window should be dedicated for terminal, not allow opening a file

[shaozi]

When the terminal window is focused, and a user clicks a tab, the buffer of the tab will take the terminal window over. This is not what a user normally expected. I am not sure if ToggleTerm can do this, I will put it up here for discussion.

See screenshot for the explanation if the issue.

[jimafisk]

Related issue posted in bufferline: akinsho/bufferline.nvim#935

[jimafisk]

@shaozi I adjusted bufferline to accomplish this: akinsho/bufferline.nvim#935 (comment)