First attempt at unicodifying MCField, MCParagraph, MCBlock, MCLine

Only MCParagraph is fully converted to Unicode and bugs remain.
Fix a layout bug caused by MCBlock thinking the text was not unicode
Fix a bug causing fonts to be ignored in certain situations
Rename MCFiltersEvalUniEncode and MCFiltersEvalUniDecode

The overloads of this function have been re-named to better reflect their operation {To,From}{Native,Encoding}
[[ Bug 10932 ]] Added override to MCField::settextatts to allow preventing relayout.
[[ Bug 10932 ]] Added handling for P_UNDEFINED to MCField::settextatts to just force a relayout in the same way as if a prop was set.
[[ Bug 10932 ]] Used new settextatts() semantics to speed up setting P_FLAGGED_RANGES.

Conflicts:
	engine/src/fields.cpp
Merge branch 'refactor-syntax_unicode_kupdown' into refactor-syntax_unicode_fieldtext

Conflicts:
	engine/src/field.cpp
	engine/src/fieldf.cpp
	engine/src/object.cpp
Modify MCBlock and MCLine to use StringRefs throughout
Remove MCParagraph::nativizetext()
Remove text_buffer_t

The corresponding header file is no longer needed and has been removed
Improvements and fixes to field text handling

Removed some more instances of 16-bit counters being used for string lengths.
Lots of bug fixes
Ensure Unicodeness is preserved when saving block-less paragraphs
Replacement of various integers by findex_t, where appropriate
Merge remote-tracking branch 'upstream/refactor-syntax_unicode' into refactor-syntax_unicode_fieldtext

Conflicts:
	engine/src/block.h
	engine/src/exec-interface2.cpp
	engine/src/fieldstyledtext.cpp
	engine/src/object.cpp
Fixes for building on Linux
Merge branch 'refactor-syntax_unicode_kupdown' into refactor-syntax_unicode_fieldtext
Merge remote-tracking branch 'upstream/refactor-syntax_unicode' into refactor-syntax_unicode_fieldtext

Conflicts:
	engine/src/block.cpp
Merge remote-tracking branch 'fraser/refactor-syntax_unicode_fieldtext' into refactor-syntax_unicode_paragraph_attr

Conflicts:
	engine/src/block.cpp
[[ RefactorSyntax ]] Work on refactoring field chunk properties.
[[ RefactorSyntax ]] More work on paragraph attrs refactoring.