fix: handle new Screen Recording permission on Catalina (closes #29)

This is a light rework of the work from #33

alt-tab-macos/ui/Application.swift

@@ -31,6 +31,7 @@ class Application: NSApplication, NSApplicationDelegate, NSWindowDelegate, NSCol
     }
 
     func applicationDidFinishLaunching(_ aNotification: Notification) {
+        ensureScreenRecordingCheckboxIsChecked()
         ensureAccessibilityCheckboxIsChecked()
         updateThumbnailMaxSize()
         makeStatusBarItem()
@@ -106,7 +107,16 @@ class Application: NSApplication, NSApplicationDelegate, NSWindowDelegate, NSCol
 
     func ensureAccessibilityCheckboxIsChecked() {
         if !AXIsProcessTrustedWithOptions(["AXTrustedCheckOptionPrompt": true] as CFDictionary) {
-            debugPrint("The user hasn't checked the accessiblity permission checkbox; please authorize and re-launch\nSee https://help.rescuetime.com/article/59-how-do-i-enable-accessibility-permissions-on-mac-osx")
+            debugPrint("Before using this app, you need to give permission in System Preferences > Security & Privacy > Privacy > Accessibility.\nPlease authorize and re-launch.\nSee https://help.rescuetime.com/article/59-how-do-i-enable-accessibility-permissions-on-mac-osx")
+            NSApp.terminate(self)
+        }
+    }
+
+    func ensureScreenRecordingCheckboxIsChecked() {
+        let firstWindow = cgWindows()[0]
+        let windowImage = CGWindowListCreateImage(.null, .optionIncludingWindow, firstWindow[kCGWindowNumber] as! CGWindowID, [.boundsIgnoreFraming, .bestResolution])
+        if windowImage == nil {
+            debugPrint("Before using this app, you need to give permission in System Preferences > Security & Privacy > Privacy > Screen Recording.\nPlease authorize and re-launch.\nSee https://dropshare.zendesk.com/hc/en-us/articles/360033453434-Enabling-Screen-Recording-Permission-on-macOS-Catalina-10-15-")
             NSApp.terminate(self)
         }
     }