feat(bar): support swaybar mode for configuring window

Use `mode` (`waybar::Bar::setMode`) as a shorthand to configure bar
visibility, layer, exclusive zones and input event handling in the same
way as `swaybar` does.
See `sway-bar(5)` for a description of available modes.

include/bar.hpp

@@ -58,6 +58,7 @@ class Bar {
   Bar(const Bar &) = delete;
   ~Bar() = default;
 
+  void setMode(const std::string &);
   void setVisible(bool visible);
   void toggle();
   void handleSignal(int);