Older how_to floater is not set to persist in between sessions and gets stuck, ranamed new how_to into guidebook to avoid the issue

following promotion of DRTVWR-525

SL-15299 SL-15313 BUG-230802 Revert BOM fix causing issues with avatar alpha masks.

Approved-by: Andrey Kleshchev

Revert "SL-12069 Issues with bakes-on-mesh and alpha modes"
This reverts commit 5691d302.

Opening and closing single-instance floaters rapidly and multiple times could result in cleaning instance from floaterreg twice, thus removing new isntance instead of current one

Merge branch 'DRTVWR-518-ui' of https://bitbucket.org/lindenlab/viewer/commits/branch/DRTVWR-518-ui into DRTVWR-518-ui

"how_to" floater is supposed to be single instance

instead of an url function takes an uri

DRTVWR-525 merge up to 6.4.19

# Conflicts:
#	indra/newview/llfloaterfixedenvironment.cpp
#	indra/newview/skins/default/xui/en/strings.xml

following promotion of DRTVWR-515