Make FrameworkElement.Triggers support more types of triggers

[DingpingZhang]

FrameworkElement.Triggers now only accepts EventTriggers type, and we have to write a Style to add other triggers to the element, as shown below:

        <Button>
            <Button.Style>
                <Style TargetType="Button" BasedOn="{StaticResource {x:Type Button}}">
                    <Style.Triggers>
                        <Trigger Property="IsEnabled" Value="False">
                            <Setter Property="Background" Value="Gray" />
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </Button.Style>
        </Button>

I think we need a more simplified xaml syntax to add triggers (Trigger, DataTrigger, MultiTrigger and MultiDataTrigger), just like below:

        <Button>
            <Button.Triggers>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter Property="Background" Value="Gray" />
                </Trigger>
            </Button.Triggers>
        </Button>

[karelz]

@rladuca I suggest to use just 1 issue type label (although it is not clearly stated in the guide - something to fix). In this sense, each api-suggestion is implicitly an enhancement.