Fix replacing tilde in pango for text string.
Fix replacing tilde in pango for text string.

Fixes https://forum.shotcut.org/t/weird-text-rendering-when-tilde-
included-in-the-text/5637/3

This retains the special behavior for tilde for the special filename
(+string.txt) and .mpl files.

mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt