Squach commit of multiple fixes that were done recently in

release/21.08 branch.

Don't crash when calling the context menu of a rendered markdown entry.

Don't crash when all worksheets were closed and a new one is created
again.

BUG: 443177
FIXED-IN: 21.08.2

Don't create a new worksheet via the double click in the "Select the backend" dialog if the backend is not enabled.

Don't shown the "move up" and "move down" actions in the context menu of a worksheet entry if the entry is already at the very top or very bottom, respectively.

Since we don't have the undo/redo functionality yet, removing an entry by an error can result in a lost of a lot of work.
To at least remedy this problem, pop up a Yes/No-message box and ask the user to confirm.

Re-activated the action toolbar again for worksheet entries (seems to be a regression) and added short cuts for remove, move up and move down actions.

Mark the project as modified also when the entries are move or deleted and properly notify the user about the changes in the window title bar and in the tab names.

Remove the wait cursor prior to showing the message box on LaTeX errors in Jupytor notebooks.