[Mouse Cursor KCM] Improve installation workflow

• Support drag and drop install
• Open file dialog directly rather than opening a dialog with an input field and then having to click "Browse" again
• Use inline message instead of message box for "you need to restart for changes to take effect" (if applying fails)
• Port from deprecated KIO::NetAccess to using QTemporaryFile and make downloading async

Differential Revision: https://phabricator.kde.org/D12491