* New function UTF8_char_offset_to_byte_offset

Signed-off-by: drane <drane@users.sourceforge.net>
* Allow cursor movement within text entry boxes

Signed-off-by: drane <drane@users.sourceforge.net>
* UTF8_insert_char_at: fix minor off-by-one error

Fortunately it was too conservative, instead of crashing.
This made textbox cursors disappear when they contained about 64 bytes.

Signed-off-by: drane <drane@users.sourceforge.net>
* add more UTF-8 tests

Includes a check for the bug fixed by the previous patch.

Signed-off-by: drane <drane@users.sourceforge.net>
* uiTextEntryNode::draw: use UTF8_strncpyz() to copy UTF-8 string

Signed-off-by: drane <drane@users.sourceforge.net>