Implements the ability to perform a quick save and quick load in single-player.

[CCHyper]

Closes #358

This pull request implements the ability to perform a quick save and quick load in single-player (campaign and skirmish). These actions are performed by using the following new key keyboard commands;

Quick Save Perform a quick save. Default to Ctrl+N

Quick Load Load the most recent quick save (if one exists). Default to Ctrl+M

Each quick save is saved with the following description format: [Quick Save <slot number>: <mission description >]. There is a maximum of 32 available slots.