view: introduce state struct to simplify code

The state struct holds all of the state that is double-buffered and
applied through transactions. This more explicit handling simplifies
much of the code, and will allow for easier implementation of new
feature such as fullscreen.

river/Seat.zig

@@ -125,7 +125,7 @@ pub fn focus(self: *Self, _view: ?*View) void {
     // If view is null or not currently visible
     if (if (view) |v|
         v.output != self.focused_output or
-            v.current_tags & self.focused_output.current_focused_tags == 0
+            v.current.tags & self.focused_output.current_focused_tags == 0
     else
         true) {
         // Set view to the first currently visible view on in the focus stack if any