Add WindowManager::getLastSelectedWindow() to replace getMainWindow()

[Mm2PL]

Description

This is needed to fix /clearmessages. I will replace all usages of getMainWindow() in another PR.

Here is a simple fix for /clearmesages which can be used to test this.

Fixes #3353. Required to finally close issue #2406.

Patch

diff --git a/src/controllers/commands/CommandController.cpp b/src/controllers/commands/CommandController.cpp
index 9ce784ba..68eb2939 100644
--- a/src/controllers/commands/CommandController.cpp
+++ b/src/controllers/commands/CommandController.cpp
@@ -1377,18 +1377,25 @@ void CommandController::initialize(Settings &, Paths &paths)
         return "";
     });
 
-    this->registerCommand("/clearmessages", [](const auto & /*words*/,
-                                               ChannelPtr channel) {
-        auto *currentPage = dynamic_cast<SplitContainer *>(
-            getApp()->windows->getMainWindow().getNotebook().getSelectedPage());
+    this->registerCommand(
+        "/clearmessages",
+        [](const auto & /*words*/, ChannelPtr channel) -> QString {
+            auto *win = getApp()->windows->getLastSelectedWindow();
+            if (win == nullptr)
+            {
+                return "";
+            }
 
-        if (auto split = currentPage->getSelectedSplit())
-        {
-            split->getChannelView().clearMessages();
-        }
+            auto *currentPage = dynamic_cast<SplitContainer *>(
+                win->getNotebook().getSelectedPage());
 
-        return "";
-    });
+            if (auto *split = currentPage->getSelectedSplit())
+            {
+                split->getChannelView().clearMessages();
+            }
+
+            return "";
+        });
 
     this->registerCommand("/settitle", [](const QStringList &words,
                                           ChannelPtr channel) {

[Mm2PL]

I will be making changes related to closing windows to this pr soon. I just can't do them now

[pajlada]

I will be making changes related to closing windows to this pr soon. I just can't do them now

Sounds good, I will look into some of the questionable usages of getMainWIndow/getLastSelectedWindow

[pajlada]

This is fine for me - each use of the function should be weighed with possible alternatives