per-pane title bar (implements issue #1541)

[XiminLin]

What do you want to change?

Add a native per-pane title bar to the macOS app. Each pane gets top banner showing the current working directory by default. When user double clicks the banner, the title name can be changed. Clicking Esc will cancel the title name change. Enter or click elsewhere will confirm the title renaming.

Why do you want to make this change?

having multiple panes side by side, users can easily forget what they were working on each pane. Having a custom pane title allows user to quickly remember the purpose of that pane. This feature has been long requested and available in other terminal apps.

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