Utilities: Add a disk space usage analyzation program.

SpaceAnalyzer: Partially address code review changes.

- Use GUI::CommonActions::make_about_action().
- Pass large arguments by const reference instead of by value.
- Mark const functions as such.
- Add newline at end of SpaceAnalyzer.af
- Use full words instead of abbreviations in variable names.
- Use application's namespace instead of 'TreeMap'.
- move() certain assignments.
- Use member declaration initialization.
- Initialize TreeNode* member of QueueEntry.
- Rewrite find_mount_for_path to return MountInfo* instead.
- Rename ITreeMap and ITreeMapNode to TreeMap and TreeMapNode.
- Replace ext suffix with rect suffix for rectangles.

SpaceAnalyzer: Further address code review and coding style.

- Remove get prefix from accessor functions.
- Layout algorithm in its own function, with callback.
- Remove nullptr comparisons.
- Store lstat errors in error_accumulator.
- Use Rect::shatter.
- Use Rect's orientation based functions.

SpaceAnalyzer: Make sort_children_by_area const qualified.

Base/res/apps/SpaceAnalyzer.af

@@ -0,0 +1,4 @@
+[App]
+Name=SpaceAnalyzer
+Executable=/bin/SpaceAnalyzer
+Category=Utilities