app/internal/wm: rename confusing app/internal/window

Package wm (for "window manager") is a better fit for the package, and
distinguishes it from the low-level package windows for the Windows API.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/app.go

@@ -6,7 +6,7 @@ import (
 	"os"
 	"strings"
 
-	"gioui.org/app/internal/window"
+	"gioui.org/app/internal/wm"
 )
 
 // extraArgs contains extra arguments to append to
@@ -44,5 +44,5 @@ func DataDir() (string, error) {
 // require control of the main thread of the program for
 // running windows.
 func Main() {
-	window.Main()
+	wm.Main()
 }