Fix issue where homescreen applet is missing metadata

When running Plasma Bigscreen via QT_QPA_PLATFORM=wayland dbus-run-session kwin_wayland "plasmashell -p org.kde.plasma.mycroft.bigscreen", an error is shown that the component org.kde.plasma.mycroft.bigscreen.homescreen is missing metadata. A warning like this is also shown during cmake ... This commit fixes the issue by copying metadata.json from containments/homescreen/plugin to containments/homescreen/package.

This commit also adds build/ to .gitignore.