A few small adjustments to file dialogs:

a) Added Backspace key support for Tree-based file dialog.
b) Fixed issue inability to select a folder in project manager (always previous folder was selected instead).
c) Open Directory mode: changed "Open" to "Select Current Folder"
d) Block "Open" button when inappropriate content is selected (for example, file when in open folder mode, or folder when in open files mode)

scene/gui/file_dialog.h

@@ -33,7 +33,6 @@
 #include "box_container.h"
 #include "os/dir_access.h"
 #include "scene/gui/dialogs.h"
-#include "scene/gui/dialogs.h"
 #include "scene/gui/line_edit.h"
 #include "scene/gui/option_button.h"
 #include "scene/gui/tool_button.h"
@@ -117,6 +116,8 @@ private:
 
 	void _unhandled_input(const Ref<InputEvent> &p_event);
 
+	bool _is_open_should_be_disabled();
+
 	virtual void _post_popup();
 
 protected: