[Feature] Don't lose the input when changing the focus window

[naokiri]

This is a feature request found on naokiri/cskk#260

Current implementation

fcitx5-cskk/src/cskk.cpp

Line 95 in 9891505

void FcitxCskkEngine::deactivate(const InputMethodEntry &entry,

Currently resets when the IME is deativated, which loses the input.
fcitx5で別のウィンドウをクリックした時にdeactivateされ、resetでinput内容が失われます。

Wanted implementation

When losing focus and deactivated, don't reset and keep the context.
When coming back to the same context (might depend on the fcitx5 configuration what is the 'same' context), user can input continuously from the previous state.
元に戻ってきた時に状態を保持したままでいて続きから入力できるようになったら嬉しいかもしれない。