Permit right clicking patterns in pattern list as well as in the tileset / Add delete button to patterns

[ghost]

Presently the user can left click a pattern to reveal where on the tileset it is sampled from. However, in order to retrieve the right click menu, the user needs to right click the now highlighted pattern in the tileset.

This is unfortunate for two reasons.

1. The right click menu is presented slightly different from the options at the bottom of the patterns list. This is an unnecessary mental caveat -- personally I'd like to just hide the options in the patterns list, and simply right click my patterns.
2. More importantly, there is no delete button in the patterns list, so you must venture into the tileset in order to delete your pattern.

I suggest allowing right clicking in the patterns list. I furthermore suggest adding a delete button. Finally, I suggest allowing users to just hide the patterns list, and rely on right clicking.

[christopho]

Workaround for 2: you can press the delete key from the pattern list.