Make output position configurable

Implements https://github.com/YaLTeR/niri/issues/14

resources/default-config.kdl

@@ -33,6 +33,17 @@ input {
 /-output "eDP-1" {
     // Scale is a floating-point number, but at the moment only integer values work.
     scale 2.0
+
+    // Position of the output in the global coordinate space.
+    // This affects directional monitor actions like "focus-monitor-left", and cursor movement.
+    // The cursor can only move between directly adjacent outputs.
+    // Output scale has to be taken into account for positioning:
+    // outputs are sized in logical, or scaled, pixels.
+    // For example, a 3840×2160 output with scale 2.0 will have a logical size of 1920×1080,
+    // so to put another output directly adjacent to it on the right, set its x to 1920.
+    // It the position is unset or results in an overlap, the output is instead placed
+    // automatically.
+    position x=1280 y=0
 }
 
 // Add lines like this to spawn processes at startup.