Fixes for dual monitors:
-Freecol starts on the monitor where the mouse cursor is.
-Don't always reserve 32 pixels for insets, a secondary monitor can have 0.
-Explicitly maximize window if in windowed mode, since that removes the window borders on my KDE setup.
-When switching from windowed to fullscreen, go fullscreen on the window's current screen, not on the primary screen.

Internally, the gui class' "GraphicsDevice gd" var has been removed, since it changes as the window is moved between physical screens.