[API] Implement ImGuiFileDialog extension (#89)

* Initial setup for ImGuiFileDialog extension

* Need to resolve linker errors. Working on PaneFun callback.

* Remove jni callback code that is not compiling to expose linker error.

* Callbacks for paneFun not yet implemented. Linker error still present, seems that some header is being included multiple times.

* Fixed linker errors. Added example for demo.

* Add support for multiple callbacks

* Callbacks work

.gitmodules

@@ -16,3 +16,6 @@
 [submodule "include/ImGuiColorTextEdit"]
 	path = include/ImGuiColorTextEdit
 	url = https://github.com/BalazsJako/ImGuiColorTextEdit
+[submodule "include/ImGuiFileDialog"]
+	path = include/ImGuiFileDialog
+	url = https://github.com/aiekick/ImGuiFileDialog