windowmanager destructor free _mainwindow, fix save - remove const in (#36)

* windowmanager destructor free _mainwindow, fix save - remove const in
settingsPath that is being written to from ptree tree

* syntax error

* remove destructor, singleton, let OS claim the resource as no special cleanup of _mainWindow is not needed

windowmanager.cpp

@@ -10,8 +10,7 @@ namespace chatterino {
 WindowManager WindowManager::instance;
 
 WindowManager::WindowManager()
-    : _windowMutex()
-    , _mainWindow(nullptr)
+    : _mainWindow(nullptr)
 {
 }
 
@@ -103,7 +102,7 @@ void WindowManager::load()
 
 void WindowManager::save()
 {
-    const auto &settingsPath = getSettingsPath();
+    auto &settingsPath = getSettingsPath();
     boost::property_tree::ptree tree;
 
     // Create windows array