Mention backend name in "Install" button to make the source super clear
This allows the backend to be really really clear to the user, even if they're not familiar with the logos and don't think to hover the cursor over it to see the tooltip. In fact with this commit, the tooltip is removed because the name is now visible by default in the button name, so it's no longer needed.
The backend icon can not yet be displayed inside the Install button due to https://bugs.kde.org/show_bug.cgi?id=433433.
This is an alternative to !83 (closed)
BUG: 433370 FIXED-IN: 5.22