LibGUI: Shift+Tab unindents line

Previously, pressing Shift+Tab would indent the line if no selection was
given. While with a selection, it would be unindented. With this change,
pressing Shift+Tab with no selection unindents the current line.

For this, add unindent_line() helper function. This function unindents the
current line by at most one tab width if it starts with whitespace,
regardless of cursor position.

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -918,6 +918,11 @@ void TextEditor::keydown_event(KeyEvent& event)
                 indent_selection();
                 return;
             }
+        } else {
+            if (event.modifiers() == Mod_Shift) {
+                unindent_line();
+                return;
+            }
         }
     }
 
@@ -1050,6 +1055,18 @@ void TextEditor::unindent_selection()
     }
 }
 
+void TextEditor::unindent_line()
+{
+    if (current_line().first_non_whitespace_column() != 0) {
+        auto const unindent_size = current_line().leading_spaces() < m_soft_tab_width ? current_line().leading_spaces() : m_soft_tab_width;
+        auto const temp_column = m_cursor.column();
+
+        execute<UnindentSelection>(unindent_size, TextRange({ m_cursor.line(), 0 }, { m_cursor.line(), line(m_cursor.line()).length() }));
+
+        set_cursor({ m_cursor.line(), temp_column <= unindent_size ? 0 : temp_column - unindent_size });
+    }
+}
+
 void TextEditor::delete_previous_word()
 {
     TextRange to_erase(document().first_word_before(m_cursor, true), m_cursor);

Userland/Libraries/LibGUI/TextEditor.h

@@ -158,6 +158,7 @@ public:
     bool is_indenting_selection();
     void indent_selection();
     void unindent_selection();
+    void unindent_line();
 
     Function<void()> on_change;
     Function<void(bool modified)> on_modified_change;