source_group: Add options create groups matching directory tree

Add `TREE` and `PREFIX` arguments to enable this behavior.

Tests/SourceGroups/CMakeLists.txt

@@ -30,6 +30,17 @@ source_group(Base\\Sub1\\Base FILES bar.c)
 # a group without files, is currently not created
 source_group(EmptyGroup)
 
+set(root ${CMAKE_CURRENT_SOURCE_DIR})
 
-add_executable(SourceGroups main.c bar.c foo.c sub1/foo.c sub1/foobar.c baz.c README.txt)
+set(tree_files_without_prefix ${root}/sub1/tree_bar.c
+                              ${root}/sub1/tree_baz.c
+                              ${root}/sub1/tree_subdir/tree_foobar.c)
 
+set(tree_files_with_prefix ${root}/tree_foo.c)
+
+source_group(TREE ${root} FILES ${tree_files_without_prefix})
+
+source_group(TREE ${root} PREFIX tree_root/subgroup FILES ${tree_files_with_prefix})
+
+add_executable(SourceGroups main.c bar.c foo.c sub1/foo.c sub1/foobar.c baz.c
+                            ${tree_files_with_prefix} ${tree_files_without_prefix} README.txt)

Tests/SourceGroups/main.c

@@ -5,10 +5,17 @@ extern int bar(void);
 extern int foobar(void);
 extern int barbar(void);
 extern int baz(void);
+extern int tree_foo(void);
+extern int tree_bar(void);
+extern int tree_foobar(void);
+extern int tree_baz(void);
 
 int main()
 {
   printf("foo: %d bar: %d foobar: %d barbar: %d baz: %d\n", foo(), bar(),
          foobar(), barbar(), baz());
+
+  printf("tree_foo: %d tree_bar: %d tree_foobar: %d tree_baz: %d\n",
+         tree_foo(), tree_bar(), tree_foobar(), tree_baz());
   return 0;
 }

Tests/SourceGroups/sub1/tree_bar.c

@@ -0,0 +1,4 @@
+int tree_bar(void)
+{
+  return 8;
+}

Tests/SourceGroups/sub1/tree_baz.c

@@ -0,0 +1,4 @@
+int tree_baz(void)
+{
+  return 9;
+}

Tests/SourceGroups/sub1/tree_subdir/tree_foobar.c

@@ -0,0 +1,4 @@
+int tree_foobar(void)
+{
+  return 7;
+}

Tests/SourceGroups/tree_foo.c

@@ -0,0 +1,4 @@
+int tree_foo(void)
+{
+  return 6;
+}