Context sensitive help for inventory/equipment lists

See Oook thread for more detail.

Remove 'You may not do that now' warnings where possible

The Windows 'x' (close window) icon and the File -> Exit menu option are disabled whenever an action is in progress (e.g. inventory being shown). It would be more consistent to instead show a "Do you really want to exit Angband?" prompt. Clicking 'Yes' should cancel out action(s) in progress until the game can save+exit (or quit+exit).

Similarly File -> New and File -> Open are usually disabled. File -> New could (after warning) act as 'save+new character' and File -> Open as 'save+load character'.