HexEditor: Stream input files instead of keeping them in memory

To support editing of large files it is an advantage to not load the
entire file into memory but only load whatever is needed for display at
the moment. To make it work, file access is abstracted into a socalled
HexDocument, of which there two: a memory based and a file based one.
The former can be used for newly created documents, the latter for file
based editing.

Hex documents now do track changes instead of the HexEditor. HexEditor
only sets new values. This frees HexEditor of some responsibility.

Userland/Applications/HexEditor/SearchResultsModel.h

@@ -14,7 +14,7 @@
 #include <LibGUI/Model.h>
 
 struct Match {
-    int offset;
+    u64 offset;
     String value;
 };