cursors: change the default cursor theme to "breeze_cursors"

It's the default in the cursor theme KCM. So when the user selects Breeze in the KCM, "CursorTheme=" in "kcminputrc" would be empty, then kwin_wayland would interpret it as "default". So when the system does have a "default" theme and it's not Breeze, kwin would use that theme instead.

"Adwaita" is set as the fallback, as when "default" exists, it's very likely to be an alias to the former. So "Adwaita" might be a better fallback than "default".