Add wxActivityIndicator control.

This is a simple animated control indicating some program activity.

Provide native GTK+ (for > 2.20) and OS X implementations as well as a generic
one used under MSW.

Update the sample and the documentation.

docs/doxygen/mainpages/cat_classes.h

@@ -93,6 +93,7 @@ Related Overviews: @ref overview_cmndlg
 Typically, these are small windows which provide interaction with the user.
 Controls that are not static can have wxValidator associated with them.
 
+@li wxActivityIndicator: A small animated control indicating program activity
 @li wxAnimationCtrl: A control to display an animation
 @li wxControl: The base class for controls
 @li wxBitmapButton: Push button control, displaying a bitmap

docs/doxygen/mainpages/const_wxusedef.h

@@ -54,6 +54,7 @@ library:
 @itemdef{wxUSE_ABOUTDLG, Use wxAboutDialogInfo class.}
 @itemdef{wxUSE_ACCEL, Use wxAcceleratorTable/Entry classes and support for them in wxMenu, wxMenuBar.}
 @itemdef{wxUSE_AFM_FOR_POSTSCRIPT, In wxPostScriptDC class use AFM (adobe font metrics) file for character widths.}
+@itemdef{wxUSE_ACTIVITYINDICATOR, Use wxActivityIndicator class.}
 @itemdef{wxUSE_ANIMATIONCTRL, Use wxAnimationCtrl class.}
 @itemdef{wxUSE_ARTPROVIDER_STD, Use standard low quality icons in wxArtProvider.}
 @itemdef{wxUSE_ARTPROVIDER_TANGO, Use Tango icons in wxArtProvider.}

docs/doxygen/mainpages/screenshots.h

@@ -51,6 +51,7 @@ or directory, font or colour, ...) directly from the window containing them:
 These controls are considered to be less common and are defined in
 @ref page_libs_wxadv "adv" library:
 
+@appearance_brief{wxActivityIndicator,activityindicator}
 @appearance_brief{wxAnimationCtrl,animationctrl}
 @genericAppearance_brief{wxBannerWindow,bannerwindow}
 @appearance_brief{wxBitmapComboBox,bitmapcombobox}