Add show cursor boolean

[willmcgugan]

Fixes #5928

A recent change to hide the cursor in read-only mode, revealed plenty of edge cases.

• Restores cursor to read_only mode.
• Added a show_cursor which applies when read_only==True. Note that show_cursor is ignored when not in read only mode, as being able to make edits without a cursor just seems like sadism.
• When in read only mode the cursor doesn't blink, to give a subtle indication that you can't insert anything.
• When in read only mode with show_cursor==False then the cursor keys will scroll the window (i.e. the default behavior for scrollable containers).

[TomJGooding]

Thanks for revisiting this. I haven't had chance to test the code yet, but from a quick scan spotted a possible mistake in the docstring.