Speed up cursor column calculation in the status bar for long lines

[KAMI911]

The status bar position indicator computed the column number by stepping through a GtkTextIter one character at a time from the start of the current line to the cursor, calling gtk_text_iter_get_char and gtk_text_iter_forward_char in a loop. This function runs on essentially every cursor movement, including while typing, so on a very long line, such as a minified script or a large single-line JSON file, it added a noticeable delay on each keystroke because the cost scaled with the cursor's column position. The loop now fetches the text between the start of the line and the cursor with a single gtk_text_iter_get_slice call and walks the resulting plain UTF-8 string instead, which avoids the repeated iterator validation and buffer lookups of the old approach while producing the exact same column number, including the same tab-expansion handling.