[Enhancement] The dragged content should behave the same way as "Add to Notes"

[Softvision-MariusComan]

[Affected versions]:

• Firefox 61 and above

[Affected Platforms]:

• All Windows
• All Linux
• All Mac

[Prerequisites]:

• Have a new Firefox profile with the latest version of the "Firefox Notes" add-on (4.2.0dev), custom built on 05.07.2018, installed.
• Have any website opened.
• Have at least a note in the sidebar.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and create or open a note.
2. Select any text from the opened website.
3. Click and drag the selected text anywhere in the middle of the note's content.
4. Observe the dragged text position.

[Expected result]:

• The dragged text is pasted at the end of the note's content on a new paragraph.

[Actual result]:

• The dragged text is pasted at the beginning or at the end of the notes content.

[Notes]:

• This issue is reproducible with the TxP version of "Firefox Notes".
• Attached a screen recording of the issue:

[cedricium]

@vladikoff you agree? If so I can tackle this.