Enhance the custom tab name behavior

[chrcuray]

This is a feature request; I already proposed the pull request #678 to fit my need.

Expected Behavior

In the key tab, when the "rename session" is used, it is not possible to:

• update the previous custom name (the new field is empty)
• restore the default behavior.

Current Behavior

On the session tab, it is possible to give a custom name by right clicking it.
By default, the custom name is always empty so it is required to fill it.
If the user provide an empty string, the action is cancelled: the previous tab name is kept.

Possible Solution

In my pull request, I propose:

• Instead of the empty string, the custom name is filled by the existing custom name
• When the custom name is deleted (set empty), the default behavior is restored (use the terminal title)

Context

Regularly, I am executing a very long script/program in a terminal. Because the shell cannot provide the information (the script is still running), I am using the custom name to remind the context.
Example: "qterminal compilation-rev 14fe83c"

When I made an error in the custom name or I start a new long script, I would like to update the string (and not write it totally).
Example:** "qterminal compilation-rev 1e7bc6b"

When all is finished, I would like to get back the default "terminal title" without closing/opening a new tab.