Userland: static vs non-static constexpr variables

Problem:
- `static` variables consume memory and sometimes are less
  optimizable.
- `static const` variables can be `constexpr`, usually.
- `static` function-local variables require an initialization check
  every time the function is run.

Solution:
- If a global `static` variable is only used in a single function then
  move it into the function and make it non-`static` and `constexpr`.
- Make all global `static` variables `constexpr` instead of `const`.
- Change function-local `static const[expr]` variables to be just
  `constexpr`.

Userland/Applications/SpaceAnalyzer/main.cpp

@@ -9,6 +9,7 @@
 #include <AK/Queue.h>
 #include <AK/QuickSort.h>
 #include <AK/RefCounted.h>
+#include <AK/StringView.h>
 #include <AK/URL.h>
 #include <Applications/SpaceAnalyzer/SpaceAnalyzerGML.h>
 #include <LibCore/DirIterator.h>
@@ -27,8 +28,6 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
-static const char* APP_NAME = "Space Analyzer";
-
 struct TreeNode : public SpaceAnalyzer::TreeMapNode {
     TreeNode(String name)
         : m_name(move(name)) {};
@@ -253,6 +252,8 @@ static String get_absolute_path_to_selected_node(const SpaceAnalyzer::TreeMapWid
 
 int main(int argc, char* argv[])
 {
+    constexpr StringView APP_NAME = "Space Analyzer";
+
     auto app = GUI::Application::construct(argc, argv);
 
     RefPtr<Tree> tree = adopt_ref(*new Tree(""));