make approve step optional in addWeight

[hobofan]

Calling rlay.addWeight currently submits two transactions:

• approve call to set the allowance for the RlayToken contract
• Submiting the propostion weight

The first step was intended as a convenience feature, but can actually lead to problems when trying to use addWeight in parallel. It can also lower the allowance from a previously higher value, which can be an unwanted side-effect.

So the first step should be optional and also off by default (and maybe be extended to check if it is even required).

[hobofan]

414ff91 changed the behaviour to not set the allowance by default.