Do not show a random white line above the star rating widget

We don't want to inherit this from the GtkButton style.
Use blue stars for the user-ratings rather than yellow/gold
Add the origin as a keyword if it's not the majority source

Any source contributing less than 10% of the applications to the software
center is now added as a search term.
Do not fail to get popular apps if the AppStream data is invalid

One bad application in the YAML file shouldn't stop the rest from working.
Don't show empty app boxes if no popular results are available