[Lock Screen / Login] Add "reveal password button"

Allows to show the password on click, similar to how it's done in most other
password fields throughout the workspace.

Differential Revision: https://phabricator.kde.org/D3616

lookandfeel/contents/lockscreen/MainBlock.qml

@@ -54,6 +54,7 @@ SessionManagementScreen {
         focus: true
         echoMode: TextInput.Password
         enabled: !authenticator.graceLocked
+        revealPasswordButtonShown: true
 
         onAccepted: startLogin()
 

sddm-theme/Login.qml

@@ -52,6 +52,7 @@ SessionManagementScreen {
         placeholderText: i18nd("plasma_lookandfeel_org.kde.lookandfeel", "Password")
         focus: !showUsernamePrompt || lastUserName
         echoMode: TextInput.Password
+        revealPasswordButtonShown: true
 
         onAccepted: startLogin()