Fix cursor position issue with astral symbols

Fix cursor position issue with astral symbols

In case of astral symbols like Devanagari matras multiple wide-char codepoints
occupy a single position/column. Positioning the cursor wrt. the actual number
of wide-characters in a wide-char string gets "visually incorrect".  The trick
is to calculate the correct number of columns needed to represent a fixed-size
wide-character string.

Relevant man pages:

1. wcswidth(3)
2. wctomb(3)
3. mblen(3)

Interesting links:

1. https://www.gnu.org/software/libc/manual/html_node/Non_002dreentrant-Character-Conversion.html
2. https://www.gnu.org/software/libc/manual/html_node/Shift-State.html
3. https://10hash.com/c-programming/uchar/
4. https://mathiasbynens.be/notes/javascript-unicode#accounting-for-astral-symbols

Example file names for test:

1. Malgudi Days - मालगुडी डेज - E05. Swami and Friends - स्वामी और उसके दोस्त (Part 5)
2. Eso eso aamar ghare eso ♫ এসো এসো আমার ঘরে এসো ♫ Swagatalakshmi Dasgupta
3. Führer