Improve title toolbar on Windows and Linux

[hojberg]

On linux the window titlebar is not styled in the same way as on mac and has a secondary bar below for the various app menus. There should be only 1 titlebar and no additional "file menu bar". This means we should only show the app menu on mac and probably put the theme picker somewhere else.

Additionally, on Linux, the window titlebar doesn't have dragging enabled, so the user can't drag the window.