[debugger] Add an option 'Strings'

A new checkbox has been added. When it is checked, the range set by
the 'erl' flag '+pc' is used for determining when to print lists of
integers as strings. When it is unchecked, integer lists are never
printed as strings.

A minor incompatibility: settings saved by Erlang R16B01
or later cannot be read by Erlang R16B or earlier.
Fix a test case

The application/module 'gs' is not available on all platforms.
Use erlang:demonitor(Ref, [flush]) where applicable
Encode Erlang source files with non-ascii characters in UTF-8

To ensure that 'master' compiles when we merge 'maint' to it,
regardless of which encoding is default in 'master', all source
files with non-ascii characters *must* have the encoding specified.
Merge branch 'bjorn/fix-encoding/OTP-11041' into maint

* bjorn/fix-encoding/OTP-11041:
  Encode Erlang source files with non-ascii characters in UTF-8
Convert XML files to UTF-8, where needed
Convert XML files to UTF-8
Fix unmatched_returns warnings in STDLIB and Kernel