Systray: Add settings for which screen to display on

This makes it possible to have the systray only visible on the primary screen,
which is the new default value. This also makes it possible to configure
directly on which screen the systray should be visible.

This breaks the API in the sense that people who use "the old method" to
configure the systray's screen possibly don't have a systray.

Fixes: https://github.com/awesomeWM/awesome/issues/724
Signed-off-by: Uli Schlachter <psychon@znc.in>

awesomerc.lua

@@ -208,7 +208,7 @@ for s = 1, screen.count() do
         { -- Right widgets
             layout = wibox.layout.fixed.horizontal,
             mykeyboardlayout,
-            s == 1 and wibox.widget.systray(),
+            wibox.widget.systray(),
             mytextclock,
             mylayoutbox[s],
         },

lib/wibox/widget/systray.lua

@@ -7,7 +7,10 @@
 
 local wbase = require("wibox.widget.base")
 local beautiful = require("beautiful")
-local capi = { awesome = awesome }
+local capi = {
+    awesome = awesome,
+    screen = screen
+}
 local setmetatable = setmetatable
 local error = error
 local abs = math.abs
@@ -18,8 +21,20 @@ local instance = nil
 local horizontal = true
 local base_size = nil
 local reverse = false
+local display_on_screen = "primary"
+
+local function should_display_on(s)
+    if display_on_screen == "primary" then
+        return s == capi.screen.primary
+    end
+    return s == display_on_screen
+end
 
 function systray:draw(context, cr, width, height)
+    if not should_display_on(context.screen) then
+        return
+    end
+
     local x, y, _, _ = wbase.rect_to_device_geometry(cr, 0, 0, width, height)
     local num_entries = capi.awesome.systray()
     local bg = beautiful.bg_systray or beautiful.bg_normal or "#000000"
@@ -49,7 +64,11 @@ function systray:draw(context, cr, width, height)
                          base, is_rotated, bg, reverse, spacing)
 end
 
-function systray:fit(_, width, height)
+function systray:fit(context, width, height)
+    if not should_display_on(context.screen) then
+        return 0, 0
+    end
+
     local num_entries = capi.awesome.systray()
     local base = base_size
     local spacing = beautiful.systray_icon_spacing or 0
@@ -98,6 +117,15 @@ function systray:set_reverse(rev)
     reverse = get_args(self, rev)
 end
 
+--- Set the screen that the systray should be displayed on.
+-- This can either be a screen, in which case the systray will be displayed on
+-- exactly that screen, or the string `"primary"`, in which case it will be
+-- visible on the primary screen. The default value is "primary".
+-- @tparam screen|"primary" s The screen to display on.
+function systray:set_screen(s)
+    display_on_screen = get_args(self, s)
+end
+
 local function new(revers)
     local ret = wbase.make_widget()
 
@@ -113,6 +141,11 @@ local function new(revers)
         ret:emit_signal("widget::layout_changed")
         ret:emit_signal("widget::redraw_needed")
     end)
+    capi.screen.connect_signal("primary_changed", function()
+        if display_on_screen == "primary" then
+            ret:emit_signal("widget::layout_changed")
+        end
+    end)
 
     return ret
 end