Editor: Allow pasting text to image caption

[aduth]

Fixes #1658

This pull request seeks to resolve an issue where pasting while the current selection is within an image caption would cause the caption to be added to the paragraph following the image, not the image caption itself.

Testing instructions:

Verify that you can paste text into a caption, including when selecting the entire caption via double- or triple-click.

1. Navigate to the Calypso post editor
2. Select a site, if prompted
3. Insert an image to post contents
4. If not captioned already, select the image, and click Add Caption in the inline image toolbar
5. Place cursor within or select caption contents
6. Paste text
7. Note that clipboard contents are inserted within the caption itself

Follow-up tasks:

We should pursue one of the following:

• Propose change to TinyMCE paste plugin to use editor.selection.setContent when pasting, in place of editor.insertContent
• Submit patch to WordPress core Trac with equivalent changes as implemented here

[lancewillett]

CC @azaozz for follow-up tasks.

[aduth]

Related: https://core.trac.wordpress.org/ticket/36211

[lancewillett]

Looks like this is still pending inclusion in core WordPress of the proposed change.