[GTK] The new web inspector can't be docked again once undocked
https://bugs.webkit.org/show_bug.cgi?id=121544

Reviewed by Martin Robinson.

Use a GtkHeaderBar for the inspector window to add dock buttons
when building with GTK+ >= 3.10.

* UIProcess/WebInspectorProxy.cpp:
(WebKit::WebInspectorProxy::WebInspectorProxy):
* UIProcess/WebInspectorProxy.h:
* UIProcess/gtk/WebInspectorProxyGtk.cpp:
(WebKit::WebInspectorProxy::dockButtonClicked):
(WebKit::WebInspectorProxy::createInspectorWindow):
(WebKit::WebInspectorProxy::updateInspectorWindowTitle):
(WebKit::WebInspectorProxy::platformInspectedURLChanged):
(WebKit::WebInspectorProxy::platformAttachAvailabilityChanged):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174327 268f45cc-cd09-0410-ab3c-d52691b4dbfc