Explicitly ask for accessibility permissions when pasting

The default behavior when AppKit asks for access the first time you
paste proves to be error-prone when upgrading to newer versions of
Maccy.

Closes #60 and #70.

Maccy/Clipboard.swift

@@ -41,6 +41,8 @@ class Clipboard {
 
   // Based on https://github.com/Clipy/Clipy/blob/develop/Clipy/Sources/Services/PasteService.swift.
   func paste() {
+    checkAccessibilityPermissions()
+
     DispatchQueue.main.async {
       let vCode = UInt16(kVK_ANSI_V)
       let source = CGEventSource(stateID: .combinedSessionState)
@@ -77,4 +79,9 @@ class Clipboard {
 
     changeCount = pasteboard.changeCount
   }
+
+  private func checkAccessibilityPermissions() {
+    let options: NSDictionary = [kAXTrustedCheckOptionPrompt.takeRetainedValue(): true]
+    AXIsProcessTrustedWithOptions(options)
+  }
 }