Change InputMethod::show() and InputMethod::hide() to not change active state.

Qt usually request InputMethod::hide() upon unfocus, but
InputMethod::show() is actually never called if focus transfer is done
by keyboard, which leads to a permanent disabled input method state.

It can be easily reproduced with a window with two text field, e.g.
QFileDialog, by pressing tab to switch the focused widget.

The semantics of hide/show should not deactivate the input method.
Instead, it should simply hide/show the input panel. Also it should not
be a hard request for input method to permanently hide the window. When
input method asked to show it again, the input panel window should be
shown.