plugins/buttonrebinds: remove modifiers before keysym conversion
QXkbCommon::toKeysym can't handle modifiers in key.
plugins/buttonrebinds: choose keysym based on KeypadModifier status
Use keysyms from the keypad if and only if KeypadModifier is set.
Del correctly working depends on upstream https://codereview.qt-project.org/c/qt/qtbase/+/546610.
Del still does not work since QXkbCommon::toKeysym sometimes only returns one key sym even when there are multiple matching key syms. (https://github.com/qt/qtbase/blob/dev/src/gui/platform/unix/qxkbcommon.cpp#L455)
E.g., the Qt::Key_Delete maps to both XKB_KEY_Delete and XKB_KEY_Clear, but QXkbCommon::toKeysym only returns XKB_KEY_Clear...
Edited by Yifan Zhu