kconfig: qconf: fix the popup menu in the ConfigInfoView window

I do not know when ConfigInfoView::createStandardContextMenu() is
called.

Because QTextEdit::createStandardContextMenu() is not virtual,
ConfigInfoView::createStandardContextMenu() cannot override it.
Even if right-click the ConfigInfoView window, the "Show Debug Info"
menu does not show up.

Build up the menu in the constructor, and invoke it from the
contextMenuEvent().

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>