client: Allow QWaylandInputContext to accept composed key combinations

At the moment, we are forcing user to choose to either compose or use the text-input channel. This patch brings some of the QComposeInputContext functionality in order to let applications understand dead key combinations like they are supposed to.

Having it in QWaylandInputContext rather than in QWaylandInputDevice should solve the problems 3aedd012 had with 57c4af2b, because we are doing it in the input context rather than before. This way, if the user is overriding the input method (e.g. by setting QT_IM_MODULE), all the key strokes will still be properly forwarded to the module to use.

This in turn allows us to solve https://bugs.kde.org/show_bug.cgi?id=411729 and https://bugs.kde.org/show_bug.cgi?id=405388 since we don't need to choose anymore between physical and virual keyboards anymore.

Pick-to: 5.15 Change-Id: I8601f5d7ae21edf4b3a1191fa75877286e505588 Reviewed-by: David Edmundson davidedmundson@kde.org

BUG: 405388 Original ticket: https://codereview.qt-project.org/c/qt/qtwayland/+/338196