Text fading in a clip in Shotcut
It has been a long time since you asked the question. Maybe you have found your answer to your question in mean time. I am going to answer it anyway for you and for all others that are using this video editor.

I am using Shotcut in version 17.03.02.

There are probably more elegant and simple ways of doing this. The way I like to do it create a SVG file with the text I want (for example in Inkscape). I import this SVG into Shotcut and put in a separate video track above the main track and leave compositing enabled. Then I add fade-in and fade-out video filters to the SVG-Text video and enable the option "Adjust opacity instead of fade with black" for both filters. You can also adjust the duration of the filter/filters.

So to sum up:


create svg file with your text
add a video track with the svg file 
leave compositing on
add fade-in/out filters and adjust duration
check "Adjust opacity instead of fade with black" in both fade filters


Enjoy!