Image preview side pane

The default file picker (probably GTK file picker) contains an image preview sidebar. This is currently missing from thunar, and doesnt seem too complicated to implement as tumblerd already generates large thumbnails which are more than suitable for the task.

Simplified principle of action:

• Toggle option to enable disable preview side pane.
• When an image is selected the tumbler generated large thumbnail gets displayed in the preview pane.
• If a thumbnail does not exist it is generated by tumblerd and then displayed
• If tumblerd cannot generate a thumbnail or the file is not of a type that can display a preview a placeholder image (probably a ? is displayed)

I'd be happy to try and work on this myself, but I am not familiar enough with the codebase, so any tips on where to start with this are appreciated!