[API] Add ImGuiContext struct and ability to switch Dear ImGui contexts (#65)

Struct itself has no fields or methods. It can be used for context switching though.

* Implement ImGuiContext

* Move ImGuiContext from imgui to imgui.internal, minor bugfixes

resolves #17

imgui-binding/src/main/java/imgui/internal/ImGuiContext.java

@@ -0,0 +1,9 @@
+package imgui.internal;
+
+import imgui.binding.ImGuiStruct;
+
+public class ImGuiContext extends ImGuiStruct {
+    public ImGuiContext(final long ptr) {
+        super(ptr);
+    }
+}