a11y: add option to start dwelling (automatic click after timeout)

Ubuntu is shipping software to provide automatic clicks with its default installation for a few years already. The name of the software providing (among others) dwell clicks is mousetweaks.

It would be great if the unity-greeter provided a way for users, that are not able to click with a hardware button, to enable the dwell click offered by the mousetweaks software.

Several weeks ago, there was a little thread about (among others) how dwelling could be enabled in the unity-greeter on the ubuntu devel discussion list. Instead of trying to reproduce its content here, let me simply point you to a relevant email:
https://lists.ubuntu.com/archives/ubuntu-devel-discuss/2011-June/012709.html

The following has been added later:

Such an option would not only be useful for the users that need dwelling, but it should be implemented also for the following reason:

Let's imagine now a user that unwillingly started the dwell click by using the appropriate dwellable button on the onscreen keyboard named onboard, and closes onboard afterwards. Because of unity-greeter remembering the state of its options across logins, at the next login, he will see the system performing clicks automatically and might not be aware that he started the dwell click during the preceding session. (Please, don't solve the problem by not making it remember anymore the state of the options across logins; I would consider it as a regression.)

I assume that if there was a way to directly start onboard in the unity-greeter, this would automatically also make the user aware of the existence of the dwell click.

For the moment, the dwell click is only reachable through the onscreen keyboard onboard, but it is not onboard that offers the dwell click feature. In fact, the dwell click feature is provided by the mousetweaks package; onboard only provides an UI that is able to control the dwell click.

One final remark: Onboard cannot be seen as a solution to the problem to start the dwell click in unity-greeter, because the user has to perform a click that is not a dwell click to open Onboard that provides the dwellable button to start the dwell click.