Import recipes by drag-and-drop and copy/paste

[cydanil]

Description

This PR adds the import of recipes via drag and drop, or copy/paste.

Files, urls, and text can now be dragged or pasted in the main window to trigger imports.

To make the functionalities complete, the export of recipes as text from drag is also added:

How Has This Been Tested?

This was tested by each of the following:

	URL	Files	Plain text
Paste	✓	✓	✓
Drop	✓	✓	✓

Screenshots (if appropriate):

An example of drag and drop import, more visual than pasting:

The export of plain text recipe via dragging, similar to copying:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.