Button/Function to insert a picture

[egolus]

in general we sometimes want to have a picture in the rendered html, for example a screenshot

to achieve this, we need some steps:

• we need to take a screenshot
• maybe we want to crop it and add some markers
• we need to save the file to disk
• we need to insert the filepath inside some markup in the notebook at the cursorposition

as a simple approach we could just implement the last step and do the others outside of noteorganiser

[baudren]

I could see this as a first, simple step:

• add a "insert picture" line in right-click on the text editor
• and an icon on the toolbar
• both would open a simple two field dialog: file-browser box and caption name. This would insert the picture with proper markdown tags inside the document, at the cursor's

Once this is in, we could try to detect a drag-and-drop of a picture on top of the text area, and open the same dialog with pre-filled path?

[egolus]

done in #95