Add option 'focus-at-startup' to focus a chosen output on start (#1323)

* Implement default-output

* Fix incorrect wiki string

* Center mouse on start

* Move default-output to Output.focus-at-startup

* fixes

---------

Co-authored-by: lualeet <lualeet@null.null>
Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

wiki/Configuration:-Outputs.md

@@ -13,6 +13,7 @@ output "eDP-1" {
     transform "90"
     position x=1280 y=0
     variable-refresh-rate // on-demand=true
+    focus-at-startup
     background-color "#003300"
 }
 
@@ -164,6 +165,28 @@ output "HDMI-A-1" {
 }
 ```
 
+### `focus-at-startup`
+
+<sup>Since: next release</sup>
+
+Focus this output by default when niri starts.
+
+If multiple outputs with `focus-at-startup` are connected, they are prioritized in the order that they appear in the config.
+
+When none of the connected outputs are explicitly `focus-at-startup`, niri will focus the first one sorted by name (same output sorting as used elsewhere in niri).
+
+```kdl
+// Focus HDMI-A-1 by default.
+output "HDMI-A-1" {
+    focus-at-startup
+}
+
+// ...if HDMI-A-1 wasn't connected, focus DP-2 instead.
+output "DP-2" {
+    focus-at-startup
+}
+```
+
 ### `background-color`
 
 <sup>Since: 0.1.8</sup>