LibGUI: Fix crash in TextDocument::remove(TextRange)

Oops, we can't be appending substrings of a string we just deleted!

Fix this by building up the new line instead of trying to clear and
append in place. This works out nicely as we now do fewer document view
updates when removing a range. :^)

Libraries/LibGUI/TextDocument.h

@@ -160,6 +160,7 @@ public:
     const u32* codepoints() const { return m_text.data(); }
     size_t length() const { return m_text.size(); }
     void set_text(TextDocument&, const StringView&);
+    void set_text(TextDocument&, Vector<u32>);
     void append(TextDocument&, u32);
     void prepend(TextDocument&, u32);
     void insert(TextDocument&, size_t index, u32);