ProjectMode: Set only maximum width instead of fixed width

Seems to be a improvement in resizing behavior.

Task-number: QTCREATORBUG-11286
Change-Id: Iac19a76265c98861e4da9d23c3e5fb3a36f0386e
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/projectexplorer/projectwindow.cpp

@@ -120,7 +120,7 @@ PanelsWidget::PanelsWidget(QWidget *parent) :
 {
     // We want a 900px wide widget with and the scrollbar at the
     // side of the screen.
-    m_root->setFixedWidth(900);
+    m_root->setMaximumWidth(900);
     m_root->setContentsMargins(0, 0, 40, 0);
 
     QPalette pal = m_root->palette();