Change HTextField borders, SignIn disable buttons

By default now, fields have a transparent border if not focused and
accent-color border if focused.
Set `bordered: false` for no border at all.

The register and forgot buttons on the SignIn screen are now disabled,
since their functions aren't implemented yet.

src/qml/Pages/SignIn.qml

@@ -17,9 +17,9 @@ Item {
         enterButtonTarget: "login"
 
         buttonModel: [
-            { name: "register", text: qsTr("Register") },
-            { name: "login", text: qsTr("Login") },
-            { name: "forgot", text: qsTr("Forgot?") }
+            { name: "register", text: qsTr("Register"), enabled: false },
+            { name: "login", text: qsTr("Login"), enabled: true },
+            { name: "forgot", text: qsTr("Forgot?"), enabled: false }
         ]
 
         buttonCallbacks: ({