Modify custom tab name behaviour

[chrcuray]

Modify custom tab name behavior:
use available session name during session edit (was empty)
restoring to terminal title when custom name is empty (was not possible)

After comments on the #678 , the change was rewritten and the described use-case has been considered.

[tsujan]

I found time to have a quick look (without testing it):

• For whatever reason, the original code removes the tab icon only when the new text is set. Please keep that behavior in your PR.
• Now that you're at it, please exclude the trailing and leading spaces from text. Particularly, we don't want a name like ; do we?

[chrcuray]

Hello Tsujan,
For the icon, I remove it at each tab text change. Because I do not know how to use/test icons, I am not sure it is what you suggested.
About spaces, I didn't only removed tailing and leading spaces with the "trimmed" method. I prefered to use "simplified" to also removed duplicated spaces.

[tsujan]

Thanks! I can't find any problem in it now. Let's give it to git users…