Add ubuntu-sdk-qmake-extras content to focal images

This adds the content of ubuntu-sdk-qmake-extras to the focal docker images, because the package was removed from Ubuntu repos. It seems to be commonly used by QMake based UT apps.

To be more precise, this only adds ubuntu-click.prf, because we already had a slightly patched version of ubuntu-click-tools.prf in our images before. The diff to the one from Ubuntu repos is:

97c97
<                 $${target_name}.commands =cd $$_PRO_FILE_PWD_/po && LC_ALL=C intltool-update -d $$lang -g $${UBUNTU_TRANSLATION_DOMAIN} -x && mkdir -p $$shadowed($$_PRO_FILE_PWD_)/po && touch $${update_target_file}
---
>                 $${target_name}.commands =mkdir -p $$shadowed($$_PRO_FILE_PWD_)/po/ && cd $$_PRO_FILE_PWD_/po && LC_ALL=C intltool-update -d $$lang -g $${UBUNTU_TRANSLATION_DOMAIN} -x && touch $${update_target_file}

Tested on Guitar Tools. Without this MR, the build fails to install manifest and other click files. With the changes from MR, native and cross building work just fine.

Fixes clickable#379 (closed), contributes to clickable#277 (closed).