move String extension method to DcCore to make it reusable in DcShare

DcCore/DcCore/Extensions/String+Extensions.swift

@@ -20,4 +20,8 @@ public extension String {
         return resultString
     }
 
+    func containsCharacters() -> Bool {
+        return !trimmingCharacters(in: [" "]).isEmpty
+    }
+    
 }

deltachat-ios/Extensions/String+Extension.swift

@@ -9,10 +9,6 @@ extension String {
         return String(self[idx1..<idx2])
     }
 
-    func containsCharacters() -> Bool {
-        return !trimmingCharacters(in: [" "]).isEmpty
-    }
-
     func containsExact(subSequence: String?) -> [Int] {
         guard let searchText = subSequence else {
             return []