GisDriver: Add a function to detect small screens

This will be used for various size tweaks in following commits.
For test purposes, GIS_SMALL_SCREEN=1 can be used to override
the heuristics.
GisPage: Add a small-screen property

This will let us use this in bindings in ui files.
Hide images when we have no space for them

Hide the logo / icon we use at the top of each page, when
we are on a 'small' screen.
Drop the minimum size when we are on a small screen

We were hardcoding a minimum size of 1024x768. Stop doing that
on small screens, and maximize the window instead.
Drop use of deprecated margin-left/right
Add missing casts