gutter: split on_view_draw: get_clip_rectangle()

on_view_draw() is a quite big and complex function.
gutter: on_view_draw: use GDK_EVENT_PROPAGATE
gutter: split on_view_draw: apply_style()
gutter: split on_view_draw: begin_draw()
gutter: on_view_draw: remove dead code

In the new place where cell_area is declared, all fields of cell_area
are assigned.
gutter: split on_view_draw: end_draw()
gutter: split on_view_draw: draw_cells()
gutter: drawing: better variable names
gutter: better parameter names for get_lines()