add signal callback function for change in effect value

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00:46:51

 
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meson.build:1:0: ERROR: Meson version is 0.47.1 but project requires >= 0.48.

A full log can be found at /run/build/gstreamer/_flatpak_build/meson-logs/meson-log.txt
Error: module gstreamer: Child process exited with code 1
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WARNING: /builds/yashagrawal3/pitivi/xunit.xml: no matching files
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ERROR: Job failed: exit code 1