QtPdf: provide feature to select text within a rectangular area

// Function: FPDFText_GetBoundedText
//          Extract unicode text within a rectangular boundary on the page.
// Parameters:
//          text_page   -   Handle to a text page information structure.
//                          Returned by FPDFText_LoadPage function.
//          left        -   Left boundary.
//          top         -   Top boundary.
//          right       -   Right boundary.
//          bottom      -   Bottom boundary.
//          buffer      -   A unicode buffer.
//          buflen      -   Number of characters (not bytes) for the buffer,
//                          excluding an additional terminator.
// Return Value:
//          If buffer is NULL or buflen is zero, return number of characters
//          (not bytes) of text present within the rectangle, excluding a
//          terminating NUL. Generally you should pass a buffer at least one
//          larger than this if you want a terminating NUL, which will be
//          provided if space is available. Otherwise, return number of
//          characters copied into the buffer, including the terminating NUL
//          when space for it is available.
// Comment:
//          If the buffer is too small, as much text as will fit is copied into
//          it.
//
FPDF_EXPORT int FPDF_CALLCONV FPDFText_GetBoundedText(FPDF_TEXTPAGE text_page,
                                                      double left,
                                                      double top,
                                                      double right,
                                                      double bottom,
                                                      unsigned short* buffer,
                                                      int buflen);