Improve Online Ressource Widget

This MR contains:

• Fix for a bug: only the first source was downloaded properly, now it works for further downloads as expected too
• Fix crash of kdenlive occurring in some situations when login dialog was shown a second time
• Disable input to login dialog until it is ready to use
• Set focus to resource dialog on opening
• Improvements on status messages: use MessageBoxes instead of adding the message to the items description
• remove duplicated "overwrite existing file?" message
• Improvements on license name: append license version
• Option to add attribution text to project notes #893 (closed)