Fix corruption of scrolling.

Replace document.body.clientHeight with window.innerHeight.
Replace document.body.clientWidth with window.innerWidth.
Add i18n.html as single place for i18n texts, not otherwise part of extension

This version 7 fixes severe bug which resulted in incorrect scrollbar display

Its messages are extracted with my html_i18n_content.js devtools snippet.
Add exception handling and issue reporting to last_scroll_event_page.js.
Translate onInstalled messages to German.