Implement multiple-forms

[zwcloud]

old finished tweaks

• [x] Move methods inside Form.GUI.cs to Application: NewFrame, EndFrame, Render and Log.
• [x] Make all forms (including main form) update and render properly
  • [x] Implement IWindow.Opacity
  • [x] Update window movement implementation
  • [x] issue: viewport resizing not invoked
  • [x] Implement viewport resizing for rendering, especially for text rendering
• [x] Inline Window.Render and re-organize MeshBuffer in Form level
  • [x] Move MeshBuffer from Window to Form
  • [x] Adjust MeshBuffer usage in Form: don't clear MeshBuffer every time foreach window when build it from Window.MeshList.
• [x] Make IMGUI states GUIContext a global static object: It is a global variable at Application.ImGuiContext now.
• [x] Singe OpenGL context for multiple native windows
• [x] Win32OpengGLRenderer: decuple IRenderer from IWindow (make per native window state like hwnd, dc, viewport and scissor rect change when IRenderer::SetRenderingWindow)

New developement of multiple-forms will be based on the MultipleNativeWindowTest project.

An openGL renderer correctly renders Mesh on two win32 windows.

• [x] Two regular window should render Mesh correctly. Fixed at https://github.com/zwcloud/ImGui/commit/9802e0c9bb61212f077cce78b7f75a6fe9a997d0.
• [x] One main regular window A and one child window of A should render Mesh correctly. Fixed at https://github.com/zwcloud/ImGui/commit/340b2be34a96da6df2807e1fe325d825157fd7f9.

Misc

• [x] #73
• [x] #80 Text in popup window doesn't appear: text framebuffer size not updated
• [x] Child window has no border rendered
• [x] #81
• [ ] Mouse/Keyboard input update logic should be reconsidered

This issue blocks #69