Support focus Within Sway WM

While Wayland offers nothing general to help us support `focus` on all
window managers, WM-specific implementations are generally possible.
Sway is a tiling window manager that mimics i3, and has a reasonably
powerful CLI that can help us achieve this.

In addition to supporting `focus` for Sway, this change paves the way
for additional WM-specific Wayland functionality by adding a detection
step to wayland.kak, in a similar fashion to detection.kak.

rc/windowing/detection.kak

@@ -23,7 +23,7 @@ declare-option -docstring \
 "Ordered list of windowing modules to try and load. An empty list disables
 both automatic module loading and environment detection, enabling complete
 manual control of the module loading." \
-str-list windowing_modules 'tmux' 'screen' 'kitty' 'iterm' 'wayland' 'x11'
+str-list windowing_modules 'tmux' 'screen' 'kitty' 'iterm' 'sway' 'wayland' 'x11'
 
 hook -group windowing global KakBegin .* %{
 

rc/windowing/sway.kak

@@ -0,0 +1,51 @@
+provide-module sway %{
+
+# Ensure we're actually in Sway
+evaluate-commands %sh{
+    [ -z "${kak_opt_windowing_modules}" ] || 
+    [ -n "$SWAYSOCK" ] ||
+    echo 'fail SWAYSOCK is not set'
+}
+
+require-module 'wayland'
+
+define-command sway-focus-pid -hidden %{
+    evaluate-commands %sh{
+        pid=$kak_client_pid
+
+        # Try to focus a window with the current PID, walking up the tree of 
+        # parent processes until the focus eventually succeeds
+        while ! swaymsg [pid=$pid] focus > /dev/null 2> /dev/null ; do
+            # Replace the current PID with its parent PID
+            pid=$(ps -p $pid -o ppid=)
+
+            # If we couldn't get a PPID for some reason, or it's 1 or less, we 
+            # should just fail. 
+            if [ -z $pid ] || [ $pid -le 1 ]; then
+                echo "fail Can't find PID for Sway window to focus"
+                break
+            fi
+        done
+    }
+}
+
+define-command sway-focus -params ..1 -docstring '
+sway-focus [<kakoune_client>]: focus a given client''s window.
+If no client is passed, then the current client is used' \
+%{
+    # Quick branch to make sure we're calling sway-focus-pid from the client 
+    # the user wants to focus on.
+    evaluate-commands %sh{
+        if [ $# -eq 1 ]; then
+            printf "evaluate-commands -client '%s' sway-focus-pid" "$1"
+        else
+            echo sway-focus-pid
+        fi
+    }
+}
+complete-command sway-focus client
+
+unalias global focus
+alias global focus sway-focus
+
+}

rc/windowing/wayland.kak

@@ -49,7 +49,7 @@ define-command wayland-focus -params ..1 -docstring '
 wayland-focus [<kakoune_client>]: focus a given client''s window
 If no client is passed, then the current client is used' \
 %{
-    fail There is no way to focus another window on Wayland
+    fail 'Focusing specific windows in most Wayland window managers is unsupported'
 }
 complete-command wayland-focus client