Rhs2116 - clear stimulus parameters when the Clear button is pressed

[bparks13]

Currently, when the Clear button is pressed, all parameters in the text boxes remain even if the parameters are removed from the channel. While there is a visual update where the stimulus waveform is removed from the plot in the main window, it would probably be more clear what is happening if all of the parameter text boxes are cleared as well.

This also will add the ability to deselect all channels and then press clear to remove any parameters that aren't needed. This would be useful from a user perspective to make it easier to clear parameters if they only want to apply one specific parameter to a channel and don't want to have to click on each text box and delete the contents.

[jonnew]

Per our discussion:

1. Clearing the text: maximum amount of work to redefine parameters
2. Not clearing: amount of work maxes out at case 1, but could be less if some of the parameters are reused.

For this reason, we think we should probably not pursue this issue.