Search for common naming variants of license file

When searching for a license file, the offline installer expects it to be named ".qt-license". When a license file is downloaded from the user's account portal, it does not include the preceding "." character. Further, Chromium-based browsers add a ".txt" extension to downloaded files of "text/plain" MIME type.

Customers are often confused by the mismatch between the downloaded and expected filenames. It would be helpful if the offline installer attempted to search for a license file named "qt-license", "qt-license.txt", or ".qt-license.txt", whenever ".qt-license" is not found. The existing logic simply prompts them to buy a license.