Blender Git Loki

GPencil: Various UI name fixes

* "Flip direction" -> "Flip Direction"
* "Show drawing direction" -> "Show Directions"
* "Grease Pencil Curves" -> "Brush Curves"
(I was considering "Brush Response Curves" instead, but that seemed like too much
of a mouthful)
* "X" for removing a palette. The UI there was more similar to a standard datablock
selector, so it should use the "+X" combo instead of "+-" combo for consistency.
(Note though, presets tend to use "+-" instead - e.g. see the Render Settings)