EmoticonTextEditItem: avoid emitting dataChanged

hotspot showed that 0.15% of the KMComposerWin constructor is spent
here because setText() emits dataChanged, that's easy to remove.
2 jobs for master
Status Job ID Name Coverage
  Build
canceled #63618
build_clazy_clang_tidy

canceled #63617
linux kf5-qt5 qt5.12