HackStudio: Store known symbol declarations in ProjectDeclarations

They were previously stored inside the Locator widget. I moved them to
the globally visible ProjectDeclarations class so they can also be used
by other widgets.

Userland/DevTools/HackStudio/CMakeLists.txt

@@ -31,6 +31,7 @@ set(SOURCES
     LanguageClient.cpp
     Locator.cpp
     Project.cpp
+    ProjectDeclarations.cpp
     ProjectFile.cpp
     ProjectTemplate.cpp
     TerminalWrapper.cpp