Fix: trim whitespace in tileset name when renaming

[Praagya26]

Problem:

1. Renaming a tileset allowed whitespace-only names.
2. Leading and trailing spaces were not trimmed, resulting in inconsistent naming.

Fix:

Applied QString::trimmed() to sanitize input:
In tilesetchanges.cpp using newName.trimmed()
In tilesetdocument.cpp using name.trimmed()

This ensures:
Whitespace-only names are rejected.
Leading and trailing spaces are removed before further processing.

Result:

Prevents invalid or empty tileset names.
Improves consistency and reliability of naming behavior.

Future Work:

As suggested by @kunal649, consider introducing a CENTRALIZED VALIDATION UTILITY.
This would help enforce consistent naming rules across the codebase and reduce duplication.

[kunal649]

😅 you can actually just push extra commits to your existing branch for #4486. It’ll update that PR automatically so we can keep all these whitespace fixes in one place for Bjorn to review. It saves everyone some clicks if we group them! (let's see what bjorn's take on naming validation idea, though implementing that won't be a cup of tea )