Persist a default browse directory for file dialogs, if none is provided

* This provides a slightly more sensible default than the application working
  directory.
* The default is shared across all dialogs and open/save.

qrenderdoc/Code/Interface/PersistantConfig.cpp

@@ -287,6 +287,8 @@ bool PersistantConfig::Load(const rdcstr &filename)
 
   RENDERDOC_SetConfigSetting("Disassembly_FriendlyNaming", ShaderViewer_FriendlyNaming ? "1" : "0");
 
+  RDDialog::DefaultBrowsePath = LastFileBrowsePath;
+
   // localhost should always be available as a remote host
   bool foundLocalhost = false;
 
@@ -396,6 +398,8 @@ bool PersistantConfig::Save()
 
   RENDERDOC_SetConfigSetting("Disassembly_FriendlyNaming", ShaderViewer_FriendlyNaming ? "1" : "0");
 
+  LastFileBrowsePath = RDDialog::DefaultBrowsePath;
+
   return Serialize(m_Filename);
 }