Porting Loops by adjusting snapping behavior

[jlampel]

Loops was another tool that was useful in v3 but probably does not need to be its own special standalone tool in v4. To support the main loops functionality in v4, I think we just need to allow double tapping G in any tool to edge slide (#1357).

The one big question there is how we should handle snapping.

At the moment, Strokes uses Face Project and PolyPen uses both Face Project and Face Nearest. Loops requires Face Nearest in order to work properly.

Face Project seems to completely override Face Nearest, so I don't think we need to have both enabled at the same time in any tool.

@vxlcoder Can RF just use Face Nearest for everything by default, and only switch to Face Project internally in special circumstances when needed?

For example, use Face Project while actively drawing new geometry and then switch back to Face Nearest when the action is finished.

Are there other situations in which Face Project is needed or preferred?