LibPDF: Create basic object structure

This commit is the start of LibPDF, and introduces some basic structure
objects. This emulates LibJS's Value structure, where Value is a simple
class that can contain a pointer to a more complex Object class with
more data. All of the basic PDF objects have a representation.

Userland/Libraries/LibPDF/CMakeLists.txt

@@ -0,0 +1,7 @@
+set(SOURCES
+    Object.cpp
+    Value.cpp
+    )
+
+serenity_lib(LibPDF pdf)
+target_link_libraries(LibPDF LibC LibCore)