Added IME support for OSX examples

[actboy168]

This patch adds IME support for macOS and also fixes some input bugs, such as #2578.

Some TODO:

1. The osx/opengl example has been unusable, I can't test it and add IME support.
2. On macOS, ImGui needs to render Editing Text by itself. But this is not very important.
3. Since there is no Editing Text, the IME cannot cover the cursor. That is to say, g.FontSize cannot be subtracted here.
   

   imgui/imgui_widgets.cpp

   Line 4043 in bff7202

   g.PlatformImePos = ImVec2(cursor_screen_pos.x - 1.0f, cursor_screen_pos.y - g.FontSize);

[ocornut]

Thank you!
Some quick feedback:

• please keep the code in the same source file.
• could you later provide GIF of how this is working/behaving?

Some thoughts:

• as with clipboard, I don't rule out including this code directly in imgui.cpp to simplify integration for new users of all backends. But I don't know if it is possible without Obj-C code.
• we will later need to consider how this would work for multi-viewports (although that's not supported by imgui_impl_osx.mm at the moment only by SDL/GLFW back-ends).

[actboy168]

Because the font of the default example does not support Chinese, I used another example, but they are the same.

[actboy168]

Unlike windows, macOS needs to implement NSTextInputClient in the content view. So there is no way to implement it simply in the callback function.

[actboy168]

The reason I didn't put the code into the existing file is that I want to share the code between the metal and opengl examples. Although opengl is not supported now.

[ocornut]

Hello @actboy168,
While processing PR/issues I have now merged #3108 (commit 457d4b7)
I admit when I did so I forgot about this other PR here. #3108 looks simpler, but do you think there are features that this PR handles that #3108 did not? Trying to understand if we are missing something or if this is fully solved.
Thank you!

[actboy168]

I think it's working fine now.