Fix for "Show online help" button. (#4011)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31899 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#4092 fixed. New behaviour:
- applet initialy focused for keys,
- if we hit esc, applet gets unfocused, and we can use tab to go over the page (even pass the applet)
- if we hit enter, when applet focused, we get inside the applet again.

test in circle_tangents_accessible.html

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31900 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e