v0.4.0-demo

• Performance improved: Instead of external Curl processes, platform-independent, Qt-native methods are now used for downloading. This has roughly tripled the download speed, allowing the programme to run more smoothly and making it far less prone to freezing.
• The progress bar now correctly displays the expected number of items, or switches to an indeterminate loading animation if the number cannot be determined.
• The download button is now enabled when switching from Crawler to API mode, provided the API key field is not empty.