Previously, the Save action held a reference to the local variable for
the Save As action, which goes out of scope at the end of
`initialize_menubar()`. This meant that if you tried to Save a new
file, it would instead crash and yeet your work into the abyss.
The `view_frame_action` variable only exists for the duration of
`initialize_menubar()`, so calling it in `m_preview_window->on_close`
would crash. This fixes that by storing the action pointer inside
MainWidget. (And storing the `view_window_action` too because it felt
weird storing one and not the other.)
