LibGUI: Move GTextDocument out of GTextEditor

The idea here is to decouple the document from the editor widget so you
could have multiple editors being views onto the same document.

This doesn't work yet, since the document and editor are coupled in
various ways still (including a per-line back-pointer to the editor.)

Libraries/LibGUI/GTextDocument.cpp

@@ -0,0 +1,113 @@
+#include <LibGUI/GTextDocument.h>
+#include <ctype.h>
+
+GTextDocument::GTextDocument(GTextEditor& editor)
+    : m_editor(editor)
+{
+    m_lines.append(make<GTextDocumentLine>(m_editor));
+}
+
+void GTextDocument::set_text(const StringView& text)
+{
+    m_spans.clear();
+    m_lines.clear();
+    int start_of_current_line = 0;
+
+    auto add_line = [&](int current_position) {
+        int line_length = current_position - start_of_current_line;
+        auto line = make<GTextDocumentLine>(m_editor);
+        if (line_length)
+            line->set_text(text.substring_view(start_of_current_line, current_position - start_of_current_line));
+        m_lines.append(move(line));
+        start_of_current_line = current_position + 1;
+    };
+    int i = 0;
+    for (i = 0; i < text.length(); ++i) {
+        if (text[i] == '\n')
+            add_line(i);
+    }
+    add_line(i);
+}
+
+int GTextDocumentLine::first_non_whitespace_column() const
+{
+    for (int i = 0; i < length(); ++i) {
+        if (!isspace(m_text[i]))
+            return i;
+    }
+    return length();
+}
+
+GTextDocumentLine::GTextDocumentLine(GTextEditor& editor)
+    : m_editor(editor)
+{
+    clear();
+}
+
+GTextDocumentLine::GTextDocumentLine(GTextEditor& editor, const StringView& text)
+    : m_editor(editor)
+{
+    set_text(text);
+}
+
+void GTextDocumentLine::clear()
+{
+    m_text.clear();
+    m_text.append(0);
+}
+
+void GTextDocumentLine::set_text(const StringView& text)
+{
+    if (text.length() == length() && !memcmp(text.characters_without_null_termination(), characters(), length()))
+        return;
+    if (text.is_empty()) {
+        clear();
+        return;
+    }
+    m_text.resize(text.length() + 1);
+    memcpy(m_text.data(), text.characters_without_null_termination(), text.length() + 1);
+}
+
+void GTextDocumentLine::append(const char* characters, int length)
+{
+    int old_length = m_text.size() - 1;
+    m_text.resize(m_text.size() + length);
+    memcpy(m_text.data() + old_length, characters, length);
+    m_text.last() = 0;
+}
+
+void GTextDocumentLine::append(char ch)
+{
+    insert(length(), ch);
+}
+
+void GTextDocumentLine::prepend(char ch)
+{
+    insert(0, ch);
+}
+
+void GTextDocumentLine::insert(int index, char ch)
+{
+    if (index == length()) {
+        m_text.last() = ch;
+        m_text.append(0);
+    } else {
+        m_text.insert(index, move(ch));
+    }
+}
+
+void GTextDocumentLine::remove(int index)
+{
+    if (index == length()) {
+        m_text.take_last();
+        m_text.last() = 0;
+    } else {
+        m_text.remove(index);
+    }
+}
+
+void GTextDocumentLine::truncate(int length)
+{
+    m_text.resize(length + 1);
+    m_text.last() = 0;
+}