[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.

[Leilei332]

If we are going to pursue this at this time, we should also consider making sure the mechanism is flexible enough to support greater customizability within each mime type. Such as being able to support other user configurable syntaxes for wikitext. For example, in a tiddler tagged 'gallery' use the custom procedure 'gallery-image' and in other tiddlers use the procedure 'imagebox'.

@saqimtiaz I wonder whether this can be done by using parameters widget. For example in the original tiddler:

title: $:/core/ui/EditorToolbar/file-import
...


\procedure postImportActions()
...
\end

<$parameters import-actions=<<postImportActions>>>
...
</$parameters>

The actions can be overrided like this:

title: Example 1
condition: [tag[gallery-image]]

\procedure galleryActions()
...
\end

<$transclude $tiddler="$:/core/ui/EditorToolbar/file-import" import-actions=<<galleryActions>>/>