Restrict charset for host profile title

It's used now as a filename. Remember to also normalize pre-filled
title from imported filename by replacing illegal characters.

Passepartout-iOS/Cells/FieldTableViewCell.swift

@@ -60,6 +60,14 @@ class FieldTableViewCell: UITableViewCell {
         }
     }
     
+    var allowedCharset: CharacterSet? {
+        didSet {
+            illegalCharset = allowedCharset?.inverted
+        }
+    }
+
+    private var illegalCharset: CharacterSet?
+
     private(set) lazy var field = UITextField()
     
     weak var delegate: FieldTableViewCellDelegate?
@@ -96,6 +104,16 @@ class FieldTableViewCell: UITableViewCell {
 }
 
 extension FieldTableViewCell: UITextFieldDelegate {
+    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
+        guard let illegalCharset = illegalCharset else {
+            return true
+        }
+        guard string.rangeOfCharacter(from: illegalCharset) == nil else {
+            return false
+        }
+        return true
+    }
+    
     func textFieldDidEndEditing(_ textField: UITextField) {
         delegate?.fieldCellDidEdit(self)
     }

Passepartout-iOS/Scenes/Organizer/WizardHostViewController.swift

@@ -34,7 +34,8 @@ class WizardHostViewController: UITableViewController, TableModelHost, Wizard {
         let url: URL
 
         var filename: String {
-            return url.deletingPathExtension().lastPathComponent
+            let raw = url.deletingPathExtension().lastPathComponent
+            return raw.components(separatedBy: AppConstants.Store.filenameCharset.inverted).joined(separator: "_")
         }
 
         let hostname: String
@@ -215,6 +216,7 @@ extension WizardHostViewController {
             let cell = Cells.field.dequeue(from: tableView, for: indexPath)
             cell.caption = L10n.Wizards.Host.Cells.TitleInput.caption
             cell.captionWidth = 100.0
+            cell.allowedCharset = AppConstants.Store.filenameCharset
             cell.field.placeholder = L10n.Wizards.Host.Cells.TitleInput.placeholder
             cell.field.clearButtonMode = .always
             cell.field.returnKeyType = .done

Passepartout/Sources/AppConstants.swift

@@ -40,6 +40,16 @@ class AppConstants {
         static let providersDirectory = "Providers"
 
         static let hostsDirectory = "Hosts"
+        
+        static let filenameCharset: CharacterSet = {
+            var chars: CharacterSet = .decimalDigits
+            let english = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+            let symbols = "-_"
+            chars.formUnion(CharacterSet(charactersIn: english))
+            chars.formUnion(CharacterSet(charactersIn: english.lowercased()))
+            chars.formUnion(CharacterSet(charactersIn: symbols))
+            return chars
+        }()
     }
     
     class VPN {