Design change - add XP by default, not set.

[magroader]

The most common way to get XP is to add it. So it should be easier to add XP.

The default way to get XP should be to type in the # of XP gained, not to set it to the typed in value, so that should be what happens when you type in a value (no + required). If you want to set it, you should need to put =blah instead.