Implement Tray Icons

This commit implements the StatusNotifierItem protocol, and enables
swaybar to show tray icons. It also uses `xembedsniproxy` in order to
communicate with xembed applications.
The tray is completely optional, and can be disabled on compile time
with the `enable-tray` option. Or on runtime with the bar config option
`tray_output none`.

Overview of changes:
In swaybar very little is changed outside the tray subfolder except
that all events are now polled in `event_loop.c`, this creates no
functional difference.

Six bar configuration options were added, these are detailed in
sway-bar(5)

The tray subfolder is where all protocol implementation takes place and
is organised as follows:

tray/sni_watcher.c:
	This file contains the StatusNotifierWatcher. It keeps track of
	items and hosts and reports when they come or go.
tray/tray.c
	This file contains the StatusNotifierHost. It keeps track of
	sway's version of the items and represents the tray itself.
tray/sni.c
	This file contains the StatusNotifierItem struct and all
	communication with individual items.
tray/icon.c
	This file implements the icon theme protocol. It allows for
	finding icons by name, rather than by pixmap.
tray/dbus.c
	This file allows for asynchronous DBus communication.

See #986 #343

include/swaybar/tray/sni.h

@@ -0,0 +1,81 @@
+#ifndef _SWAYBAR_SNI_H
+#define _SWAYBAR_SNI_H
+
+#include <stdbool.h>
+#include <client/cairo.h>
+
+struct StatusNotifierItem {
+	/* Name registered to sni watcher */
+	char *name;
+	/* Unique bus name, needed for determining signal origins */
+	char *unique_name;
+	bool kde_special_snowflake;
+
+	cairo_surface_t *image;
+	bool dirty;
+};
+
+/* Each output holds an sni_icon_ref of each item to render */
+struct sni_icon_ref {
+	cairo_surface_t *icon;
+	struct StatusNotifierItem *ref;
+};
+
+struct sni_icon_ref *sni_icon_ref_create(struct StatusNotifierItem *item,
+		int height);
+
+void sni_icon_ref_free(struct sni_icon_ref *sni_ref);
+
+/**
+ * Will return a new item and get its icon. (see warning below)
+ */
+struct StatusNotifierItem *sni_create(const char *name);
+
+/**
+ * `item` must be a struct StatusNotifierItem *
+ * `str` must be a NUL terminated char *
+ *
+ * Returns 0 if `item` has a name of `str`
+ */
+int sni_str_cmp(const void *item, const void *str);
+
+/**
+ * Returns 0 if `item` has a unique name of `str` or if
+ * `item->unique_name == NULL`
+ */
+int sni_uniq_cmp(const void *item, const void *str);
+
+/**
+ * Gets an icon for the given item if found.
+ *
+ * XXX
+ * This function keeps a reference to the item until it gets responses, make
+ * sure that the reference and item are valid during this time.
+ */
+void get_icon(struct StatusNotifierItem *item);
+
+/**
+ * Calls the "activate" method on the given StatusNotifierItem
+ *
+ * x and y should be where the item was clicked
+ */
+void sni_activate(struct StatusNotifierItem *item, uint32_t x, uint32_t y);
+
+/**
+ * Asks the item to draw a context menu at the given x and y coords
+ */
+void sni_context_menu(struct StatusNotifierItem *item, uint32_t x, uint32_t y);
+
+/**
+ * Calls the "secondary activate" method on the given StatusNotifierItem
+ *
+ * x and y should be where the item was clicked
+ */
+void sni_secondary(struct StatusNotifierItem *item, uint32_t x, uint32_t y);
+
+/**
+ * Deconstructs `item`
+ */
+void sni_free(struct StatusNotifierItem *item);
+
+#endif /* _SWAYBAR_SNI_H */