Android IME support

[miscvariables]

based on checkout of sokol and sokol-samples on 2021/02/11
Android ImGui IME support. Simple Information:

No Java. Pure c/c++ android native activity.

Using JNI to call JavaVM functions, show/hide soft keyboard, create Android EditText widget in top view DecorView

Update the content between EditText and ImGui::InputText

I use 4 states to update the content: previous text in EditText(atext_prev), current text in EditText(atext), previous text in ImGui::InputText(vtext_prev), current text in ImGui::InputText(vtext). "atext" means Android-Text, "vtext" means Virtual-Text. The state machine is in sokol_app.h

Android main UI thread and sokol gl&imgui thread use Android looper to communicate. All JNI calls are done in Android main UI thread. It can avoid crash

sokol_imgui.h should pass ImGui::InputText state of current focus to sokol_app.h

sokol android seems only using c interface(no c++), so I only implement JNI c interface in sokol_app.h, JNI c++ interface has only empty functions for compiling complains.

Pure c/c++ Android app must use many tricks, the the code looks a bit dirty. There are many commented out codes in sokol_app.h, show the sucessful and unsucessful tries I ever made.

Patched files:
sokol/sokol_app.h
sokol/util/sokol_imgui.h
sokol-samples/sapp/imgui-highdpi-sapp.cc
sokol-samples/sapp/CMakeLists.txt (stripped. Contains only imgui-highdpi-sapp.cc Not necessary)
sokol-samples/libs/dbgui/dbgui.cc (only add #include imgui_internal.h, for compiling complains)
sokol-samples/tests/cpp-compile-test.cc (only add #include imgui_internal.h, for compiling complains)

[floooh]

Please see my comment here: floooh/sokol#475 (comment)

[floooh]

I've set this PR to "Draft" state too for now.