AppSwitcher: beautified close icon

Looks nicer than that horrific red X icon. If darko theme will be removed, ensure to save its nice icon-m-framework-close-thumbnail (nemo-theme-default contains only a white X, even that looks better than the original one :))

![screen2](https://f.cloud.github.com/assets/467463/181138/85b859f2-7c47-11e2-98d3-6b310e9adfcd.png)