Fix error when moving FileSystem dock

[timothyqiu]

Moving the FileSystem dock between different slots produces this error:

ERROR: Signal 'folder_color_changed' is already connected to given callable 'EditorDirDialog::reload' in that object.
at: connect (core/object/object.cpp:1431)

• For the filesystem_changed signal, it's always connected upon entering the tree and is disconnected when exiting the tree. The redundant connection check is removed.
• For the folder_color_changed signal, it's the same as filesystem_changed. This PR disconnects it when exiting the tree. So there won't be errors anymore.
• For the item_collapsed signal, it can be moved to the constructor.

[akien-mga]

Thanks!