HackStudio: Start adding a "find in files" function

Projects now contain a set of TextDocument objects. Each TextDocument
represents a member file in the project. TextDocuments may not have
their file contents loaded at all times, but they will be loaded on
demand when calling TextDocument::contents().

"Find in files" works by iterating over the documents in the project
and calling find(needle) on each one. The return value from find() is
a vector of line numbers where the needle was found.

This is obviously going to need a bunch more work. :^)

DevTools/HackStudio/Makefile

@@ -2,6 +2,7 @@ include ../../Makefile.common
 
 OBJS = \
     Project.o \
+    TextDocument.o \
     TerminalWrapper.o \
     main.o