Add ncurses::Window and ncurses::Session
Add ncurses::Element

Modify Session to reflect this
Implement ncurses::Window::draw_border
Implement ncurses::Window::refresh

Now, Window doesn't refresh after every drawing
method anymore, which caused some ugly flickering.
Store x, y, width and height in ncurses::Window
Add ncurses::Layout
Add ncurses::Vertical_stretch_layout
Add ncurses::Panel
Modify ncurses::Session to use ncurses::Panel instead of ncurses::Element
Modify ncurses::Vertical_stretch_layout to use complete height

Some of the height was lost trough integer division
Implement ncurses::Session::get_panel ()
Add ncurses::Horizontal_stretch_layout
Completely change up the ncurses::Stretch_layout
Improve color handling in ncurses::Window
Improve ncurses::Panel

Add set_border
Add error message when no layout is specified
Add prefered_size handling to ncurses::Panel
Improve Session::end
Merge branch 'ncurses'