Fixed logic issue in importance

Changed validation to send an int value out of the form before testing for valid values.
Fixed bug on saving edited project details

This was caused by a simple issue - I had a hidden field in the form that I had
forgotten to echo out. Copying from the web instructions, I had assumed that
the echo wasn't needed as it was 'hidden'...
Changed sort order for projects to 'by importance'
Changed all instances of 'priority' to 'importance'

To tidy up the naming convention, this will always be referred to as
importance.
Added project delete function

Marks project as done rather than actually deleting any data in the tables.
Project list now only shows projects with done = 0
Implemented check for deleting projects

Can now only delete projects if the actions linked to it are all marked as completed.