Introduce skeleton of taskbar button feature.

- Add classes: wxTaskBarButton and wxTaskBarButtonImpl.
- New interface in wxTopLevelWindowMSW to get its wxTaskBarButton:
    MSWGetTaskBarButton.
- A simple sample and build files under msvc.

Author: Chaobin Zhang
API for change the visibility of button in the taskbar.

- ShowInTaskbar and HideInTaskbar
- Sample of usage.

Author: Chaobin Zhang
APIs to support setting overlay icon, tooltip, thumbnail clip, progress state.

Author: Chaobin Zhang
APIs for adding a thumbnail toolbar with a specified set of buttons.

- AddThumbBarButton(wxThumbBarButton *button), ShowThumbnailToolbar().
- Add THBN_CLICKED message handler to top level window and generate a
  wxCommandEvent event.
- Sample.

Author: Chaobin Zhang
Move taskbar button from wxTopLevelWindowMSW to wxFrame.

Since there are two sub-classes of wxTopLevelWindowMSW: one is wxFrame, the
other is wxDialog. However, wxDialog have no button shown in taskbar on
windows, so the taskbar button feature should be only avaiable in wxFrame.

Author: Chaobin Zhang
Only register window message "TaskbarButtonCreated" when the first frame is created.

Author: Chaobin Zhang
Expose setting description of overlay icon for accessibility.

Author: Chaobin Zhang
API: SetThumbnailContents, for setting thumbnail contents with child window.

Author: Chaobin Zhang
Revise Show/Hide, for consistency with the other classes.

Author: Chaobin Zhang
Revise progress releted API to be more usual.

- Add PulseProgress
- Add SetProgressRange, instead of hard coding 100.

Author: Chaobin Zhang
Nit: fix code style.

Author: Chaobin Zhang
Work around the limitation of windows API when setting thumbnail toolbar buttons.

- New API: InsertThumbBarButton, AppendThumbBarButton, RemoveThumbBarButton.
- Though MSDN said that "Buttons cannot be added or deleted later, so this must
  be the full defined set. Buttons also cannot be reordered.", we can work
  around it by: when first time adding button, initialize all of the possible
  seven buttons and hide them, except the button adding. In the next time adding
  button, just show it, which can make it looks like it is added on the fly.

Author: Chaobin Zhang
Fix mingw build error of ITaskbarList3.

Author: Chaobin Zhang
Move wxTaskBarButton into core library.

Author: Chaobin Zhang
Coding style fix.

Author: Chaobin Zhang
Add doxygen docs of wxTaskBarButton.

Author: Chaobin Zhang
Allow 2-phase creation of wxThumbBarButton.

Author: Chaobin Zhang
Expose APIs that can update the behavior of wxThumbBarButton.

- void Enable(bool enable = true);
  void Disable();
- void EnableDismissOnClick(bool enable = true);
  void DisableDimissOnClick();
- void SetHasBackground(bool has = true);
- void Show(bool shown = true);
  void Hide();
- void EnableInteractive(bool interactive = true);
  void DisableInteractive();

Author: Chaobin Zhang
Add docs for several interfaces of wxThumbBarButton.

Author: Chaobin Zhang
Add new api for appending a separator to the thumbnail toolbar.

Actually, when calling AppendSeparator we are appending a disable
ThumbBarButton without background, which can simulate the behavior
of appending a separator.

Author: Chaobin Zhang