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.)
Font Editor and Theme Editor already open the dialog in system folders
(/res/fonts and /res/themes). To be fair, they do have a special folder
just for their files, but I think this is good enough if you want to
start hacking an app :^)
Additionaly, this also adds a filter to show only .gml files by default.
