Fix color buttons disappearing in windows when alpha is set (fix #10187)

Here's a fix for 10187, where colour buttons disappear completely when the colour has an alpha set on Windows. This bug is cause by user's temporary folder path being returned in short file name format by QTemporaryFile (and also by all QDir::tempPath and similar functions). Most qt functions handle short filename paths without issue, but trying to include short filename paths in stylesheets results in qt rejecting the stylesheet and not rendering the button at all. This patch uses the windows api GetLongPathName function to expand out the temporary file's path to a full long pathname, which means the stylesheet is parsed correctly and the button drawn.