help: update the Menu bar section in mainwindow.page
help: update The toolbars section in mainwindow.page
help: update the Ruler section in mainwindow.page
help: update the Timeline section in mainwindow.page
help: update the Playback toolbar section in mainwindow.page
help: update the Previewer section in mainwindow.page
help: add the Resizing components of the main window tip to mainwindow.page
help: turn the Media library section into Left pane section in mainwindow.page
help: add Middle pane section to mainwindow.page
help: add Timeline toolbar section to mainwindow.page
help: update meta info in mainwindow.page