Adjust mbstate_t to embed the state of the ICU converter.

* make room in mbstate_t for containing an ICU-converter's state
(well, in fact the whole converter object)
* adjust libroot's locale add-on to clone converters into a given
mbstate_t directly
* adjust ICUThreadLocalStorageValue to contain the converter pointer
instead of a converter-ID (if the converter is related to an
mbstate_t, it points into the mbstate_t).
* adjust users of converters to directly use converter pointers
instead of ICUConverterRef
* drop now unused ICUConverterManager and ICUConverterRef
* update gcc4 optional package

This brings our multibyte implementation into a fully working state,
both non-ascii and non-8-bit characters can now be handled normally
in the Terminal, i.e. this finally fixes #6276.

N.B.: Since the size of mbstate_t has changed, everything (including
the compiler!) needs to be rebuilt.

Start work on multibyte-support in locale backend.

* add ICUThreadLocaleStorageValue, which will be used to maintain
per-thread ICU converters
* add ICUConverterManager

Adjust mbstate_t to embed the state of the ICU converter.

* make room in mbstate_t for containing an ICU-converter's state
(well, in fact the whole converter object)
* adjust libroot's locale add-on to clone converters into a given
mbstate_t directly
* adjust ICUThreadLocalStorageValue to contain the converter pointer
instead of a converter-ID (if the converter is related to an
mbstate_t, it points into the mbstate_t).
* adjust users of converters to directly use converter pointers
instead of ICUConverterRef
* drop now unused ICUConverterManager and ICUConverterRef
* update gcc4 optional package

This brings our multibyte implementation into a fully working state,
both non-ascii and non-8-bit characters can now be handled normally
in the Terminal, i.e. this finally fixes #6276.

N.B.: Since the size of mbstate_t has changed, everything (including
the compiler!) needs to be rebuilt.

Start work on multibyte-support in locale backend.

* add ICUThreadLocaleStorageValue, which will be used to maintain
per-thread ICU converters
* add ICUConverterManager