[IDEA] Make image drag & drop mechanism pluggable

[Leilei332]

After #8486 was merged, the image & drop mechanism became hardcoded.

Although the mechanism was pluggable before, it still have problems like code duplication. So we need to update it to make it flexible enough to be customized.

Since the demand for pluggable image & drop mechanism isn't big at present, this is only a reminder for it. See the discussion in #8525.