Improve UTF-8 handling

For manipulation of UTF-8 encoded strings (insert, delete, truncate, and
size), usually conversion to wstring and back was necessary. This has
two disadvantages:
* the size of wstring is implementation defined
* the
double conversion is slow

The purpose of this patch is to add functions
to handle these tasks by a layer handling the different indexing between
codepoints and characters.

This commit applies these functions in
several places to replace the old implementations.

The byte_size_from_utf8_first() implementation will be replaced in a
separate commit as it relies on the count_leading_ones function by 8680.

Signed-off-by: aquileia <sk.aquileia@gmail.com>
Fix a wrongly passed parameter in text_box.cpp
fix signature and test for UTF-8 functions

On the advise of 8680, the unicode string was replaced by the hex
transcription of the corresponding codepoints (which he kindly
provided).
This might help to pass the overzealous encoding check of clang.

This commit also fixes a discrepancy between the signature of u8index in
declaration and definition.

Conflicts:
	src/serialization/string_utils.cpp
	src/tests/test_serialization.cpp
Get rid of utils::wide_string

utils::wide_string was a std::vector<wchar_t>
On most unix systems, this is 32 bits wide, and therefore a UCS-4 encoding.
On windows however, wchar_t is 16 bits wide, and the naive approach results
in UCS-2, which can only represent the Basic Multilingual Plane.

Most functions that used wide_string have been moved over to ucs4_string.
The Win32 API has been moved over to a specially created utf16_string instead.

Specialty tolower #ifdefs have been removed, as the towlower function has
been available since OS X 10.3, OpenBSD 3.7 and probably ancient versions of
all other BSDs too. The only issue is that on windows, the function cannot
be applied to characters outside the BMP.

Conflicts:
	src/dialogs.cpp
	src/gui/dialogs/addon_list.cpp
	src/gui/widgets/password_box.cpp
	src/gui/widgets/text.cpp
	src/gui/widgets/text_box.cpp
	src/serialization/string_utils.cpp
	src/serialization/string_utils.hpp
	src/text.cpp
	src/widgets/textbox.cpp
Prevent some possible buffer overflows
Match declaration with definition
Use existing named constants
Fix an identifier. Spotted by aquileia
Changelog entry for unicode handling improvements
Merge branch '1.12' of github.com:wesnoth/wesnoth into 1.12