Everywhere: Replace Model::update() with Model::invalidate()

Most of the models were just calling did_update anyway, which is
pointless since it can be unified to the base Model class. Instead, code
calling update() will now call invalidate(), which functions identically
and is more obvious in what it does.

Additionally, a default implementation is provided, which removes the
need to add empty implementations of update() for each model subclass.

Co-Authored-By: Ali Mohammad Pur <ali.mpfard@gmail.com>

Userland/Libraries/LibGUI/FileSystemModel.cpp

@@ -286,7 +286,7 @@ FileSystemModel::FileSystemModel(String root_path, Mode mode)
         did_update();
     };
 
-    update();
+    invalidate();
 }
 
 FileSystemModel::~FileSystemModel()
@@ -364,7 +364,7 @@ void FileSystemModel::set_root_path(String root_path)
         m_root_path = {};
     else
         m_root_path = LexicalPath::canonicalized_path(move(root_path));
-    update();
+    invalidate();
 
     if (m_root->has_error()) {
         if (on_directory_change_error)
@@ -374,7 +374,7 @@ void FileSystemModel::set_root_path(String root_path)
     }
 }
 
-void FileSystemModel::update()
+void FileSystemModel::invalidate()
 {
     m_root = adopt_own(*new Node(*this));
 
@@ -383,7 +383,7 @@ void FileSystemModel::update()
 
     m_root->reify_if_needed();
 
-    did_update();
+    Model::invalidate();
 }
 
 int FileSystemModel::row_count(const ModelIndex& index) const
@@ -665,7 +665,9 @@ void FileSystemModel::set_should_show_dotfiles(bool show)
     if (m_should_show_dotfiles == show)
         return;
     m_should_show_dotfiles = show;
-    update();
+
+    // FIXME: add a way to granularly update in this case.
+    invalidate();
 }
 
 bool FileSystemModel::is_editable(const ModelIndex& index) const