gtk: meson: Add files with public API correctly

Example symbols documented in these files are:

- gtk_snapshot_append_layout
- gtk_accessible_state_init_value
- gtk_text_mark_set_visible

gtk/meson.build

@@ -19,7 +19,6 @@ gtk_private_sources = files([
   'gdktextureutils.c',
   'gsettings-mapping.c',
   'gtkaccessibleattributeset.c',
-  'gtkaccessiblevalue.c',
   'gtkaccessiblevaluestatic.c',
   'gtkactionhelper.c',
   'gtkactionmuxer.c',
@@ -127,7 +126,6 @@ gtk_private_sources = files([
   'gtkmenutrackeritem.c',
   'gtkpanedhandle.c',
   'gtkpango.c',
-  'gskpango.c',
   'gtkpathbar.c',
   'gtkplacessidebar.c',
   'gtkplacesview.c',
@@ -147,7 +145,6 @@ gtk_private_sources = files([
   'gtkstyleanimation.c',
   'gtkstylecascade.c',
   'gtkstyleproperty.c',
-  'gtktextbtree.c',
   'gtktexthistory.c',
   'gtktextviewchild.c',
   'timsort/gtktimsort.c',
@@ -156,11 +153,13 @@ gtk_private_sources = files([
 
 # List of files that contain public API, and should be introspected
 gtk_public_sources = files([
+  'gskpango.c',
   'gtkaboutdialog.c',
   'gtkaccelgroup.c',
   'gtkaccessible.c',
   'gtkaccessiblerange.c',
   'gtkaccessibletext.c',
+  'gtkaccessiblevalue.c',
   'gtkactionable.c',
   'gtkactionbar.c',
   'gtkadjustment.c',
@@ -378,6 +377,7 @@ gtk_public_sources = files([
   'gtktestutils.c',
   'gtktext.c',
   'gtktextattributes.c',
+  'gtktextbtree.c',
   'gtktextbuffer.c',
   'gtktextchild.c',
   'gtktexthandle.c',