ProjectExplorer: Remove IRunConfigurationAspect::addToMain*Widget base

Not needed in the current use cases. The original plan was to
use a loop in the *RunConfigurationWidgets constructors, making
them less dependent on the actual RunConfiguration they serve
(and potentially have a one or few generic RunConfigurationWidget).

Change-Id: I2209213414248a1e13b50447f05e8f189620a6f1
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>