Boy this was hard.
To ease the pain of porting wireless-security to GTK4, add
a new WsFileChooserButton class that mimics the behavior of
a button that triggers a filechooser, as per the migration
guide suggests.
There were lots of GtkGrids, so the diff is particularly
horrendous. Sorry.
This needs serious testing before landing.
This means NetObject is now obsolete and can be removed.
There was a GtkSizeGroup that the mobile settings used, but it isn't clear if
this is still relevant. It should be added back later if found to be (this code
is likely to be removed to make way for a dedicated mobile panel).
