Finish to fix deprecation warnings

lib/ui/widgets/FrienshipStatusWidget.dart

@@ -72,21 +72,23 @@ class _FriendshipStatusWidgetState extends State<FriendshipStatusWidget> {
     if (widget.status.receivedRequest) {
       return Column(
         children: <Widget>[
-          RaisedButton(
+          ElevatedButton(
             child: Text(
               tr("Accept request").toUpperCase(),
               style: WhiteTextColorStyle,
             ),
-            color: Colors.green,
+            style: ButtonStyle(
+                backgroundColor: MaterialStateProperty.all(Colors.green)),
             onPressed: () => executeRequest(
                 () => _friendsHelper.respondRequest(friendID, true)),
           ),
-          RaisedButton(
+          ElevatedButton(
             child: Text(
               tr("Reject request").toUpperCase(),
               style: WhiteTextColorStyle,
             ),
-            color: Colors.red,
+            style: ButtonStyle(
+                backgroundColor: MaterialStateProperty.all(Colors.red)),
             onPressed: () => executeRequest(
                 () => _friendsHelper.respondRequest(friendID, false)),
           )

lib/ui/widgets/post_create_form_widget.dart

@@ -20,9 +20,7 @@ import 'package:image_picker/image_picker.dart';
 /// @author Pierre HUBERT
 
 const _ActiveButtonsColor = Colors.blue;
-const _ActiveButtonsTextColor = Colors.white;
 const _InactiveButtonsColor = Colors.grey;
-const _InactiveButtonsTextColor = Colors.black;
 
 class PostCreateFormWidget extends StatefulWidget {
   final PostTarget postTarget;
@@ -192,14 +190,9 @@ class _PostCreateFormWidgetState extends State<PostCreateFormWidget> {
                   // Submit post button
                   _isCreating
                       ? Container()
-                      : FlatButton(
+                      : ElevatedButton(
                           child: Text(tr("Send").toUpperCase()),
-                          onPressed: canSubmitForm ? _submitForm : null,
-                          color: _ActiveButtonsColor,
-                          textColor: _ActiveButtonsTextColor,
-                          disabledColor: _InactiveButtonsColor,
-                          disabledTextColor: _InactiveButtonsTextColor,
-                        ),
+                          onPressed: canSubmitForm ? _submitForm : null),
                 ],
               ),
             )

lib/ui/widgets/tablet_mode/conversations/conversation_window.dart

@@ -99,8 +99,7 @@ class _ConversationWindowState extends SafeState<ConversationWindow> {
         isCollapsed: _collapsed,
         body: buildErrorCard(tr("Could not load conversation information!"),
             actions: [
-              FlatButton(
-                textColor: Colors.white,
+              ElevatedButton(
                 onPressed: _refresh,
                 child: Text(tr("Try again").toUpperCase()),
               )