Add ability to delete items from REPL history

[cursive-ghost]

There have been numerous issues (#336, #971, #1325) which could have been fixed or at least worked around if it were possible to delete items from the REPL history.

[mtruyens]

I would also suggest to simultaneously cap the maximum size of the string shown in the REPL history search box. This probably avoids the slowness I currently encounter when I trigger the history search box while I happen to have a big REPL item in the history -- and in any case it would avoid the ugliness of having a huge dialog box on the screen.

[katox]

This would be very handy. Sometimes I make a typo in a longer REPL line that I need to reuse multiple times later on. It's very annoying to select the version with a typo instead of the correct one because they look almost the same (and the search filter applies to both usually).