libgimpwidgets: add GimpBusyBox

GimpBusyBox is used to show a message indicating an operation is in
progress.  It's basically just a spinner and a label, with some
styling.

We're going to use it both in app/ and in a plug-in.

devel-docs/libgimpwidgets/libgimpwidgets-docs.sgml

@@ -132,6 +132,10 @@
     <title>Index of new symbols in GIMP 2.10</title>
     <xi:include href="xml/api-index-2.10.xml"><xi:fallback /></xi:include>
   </index>
+  <index role="2.10.4" id="api-index-2-10-4">
+    <title>Index of new symbols in GIMP 2.10.4</title>
+    <xi:include href="xml/api-index-2.10.4.xml"><xi:fallback /></xi:include>
+  </index>
   <index role="deprecated" id="api-index-deprecated">
     <title>Index of deprecated symbols</title>
     <xi:include href="xml/api-index-deprecated.xml"><xi:fallback /></xi:include>

devel-docs/libgimpwidgets/libgimpwidgets-sections.txt

@@ -17,6 +17,24 @@ GIMP_IS_BROWSER_CLASS
 GIMP_BROWSER_GET_CLASS
 </SECTION>
 
+<SECTION>
+<FILE>gimpbusybox</FILE>
+<TITLE>GimpBusyBox</TITLE>
+GimpBusyBox
+gimp_busy_box_new
+gimp_busy_box_set_message
+gimp_busy_box_get_message
+<SUBSECTION Standard>
+GIMP_BUSY_BOX
+GIMP_IS_BUSY_BOX
+GIMP_TYPE_BUSY_BOX
+gimp_busy_box_get_type
+GimpBusyBoxClass
+GIMP_BUSY_BOX_CLASS
+GIMP_IS_BUSY_BOX_CLASS
+GIMP_BUSY_BOX_GET_CLASS
+</SECTION>
+
 <SECTION>
 <FILE>gimpbutton</FILE>
 <TITLE>GimpButton</TITLE>

devel-docs/libgimpwidgets/libgimpwidgets.types

@@ -4,6 +4,7 @@
 #include <libgimpwidgets/gimpcontroller.h>
 
 gimp_browser_get_type
+gimp_busy_box_get_type
 gimp_button_get_type
 gimp_cell_renderer_color_get_type
 gimp_cell_renderer_toggle_get_type