Organize examples into subfolders

[fwcd]

Given the number of examples we have, with new ones being added (such as for custom gestures, see #20), we might want to organize them into subfolders.

[fwcd]

This might make running a bit more complicated, since we would have to list them explicitly in Cargo.toml. Maybe adopting a consistent naming scheme would be a better idea (e.g. gestures_simple.rs, gestures_custom.rs, ...)