Enable uploading multiple images in demo.py

[ifsheldon]

I made a few changes to enable uploading multiple images. This should close #180.

The changes include:

• Changed the logic of upload button, image window and text input
• Added a new UI component, i.e., gallery, to show uploaded images

One remaining problem is that we cannot upload multiple images all at once, because I suspect this line may cause issues. https://github.com/Vision-CAIR/MiniGPT-4/blob/22d8888ca2cf0aac862f537e7d22ef5830036808/minigpt4/conversation/conversation.py#L133 So now, after uploading an image, a text input should follow before uploading another image.