Add IME support for Linux #215

Is your feature request related to a problem? Please describe.

#214

Describe the solution you'd like

Support linux input method: ibus, fcitx.

Describe alternatives you've considered

None.

Additional context

The source code of SDL is a good reference:

• https://github.com/SDL-mirror/SDL/blob/master/src/core/linux/SDL_ibus.c
• https://github.com/SDL-mirror/SDL/blob/master/src/core/linux/SDL_fcitx.c
• https://github.com/SDL-mirror/SDL/blob/master/src/core/linux/SDL_ime.c

About changing the position of the candidate box and the caret, you can refer to this PR:

https://github.com/lc-soft/LCUI/pull/175/files

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