Add "select all" and "clear" options for better iOS support

[kunstmusik]

As requested on Csound mailing list by Dr. B:

"Is there any chance you can modify/expand the live.csound.com app so that there are a few more buttons on the top of the screen, making it easier “select all” or “clear all”. These are pretty essential when running on an iPad or an iPhone?

It’s almost impossible to “select all” or clear the screen on an iPhone (there might be some magical key/finger combo but I don’t know it. The keyboard pops up for typing but..."

Last time tested, CodeMirror as a whole does not do well on iOS, nor do Ace or other code editor components. Needs further research as a whole for iOS.