Disable Toggle Instruction Step Mode command using activity support.

[raghucssit]

Eclipse platform has a limitation:
If a property tester is not loaded then it shows the contribution item by default. For example Toggle Instruction Step Mode command is contributed at Main Menu -> Run menu and Main toolbar. When we start the eclipse if the CDT debug UI plugin is not loaded then this command is visible by default.

This unexpected show up of the menu item has made some eclipse users uncomfortable.

We had tried to fix in other way in the past but they were not technically/functionally possible due to legacy reasons.

How we got this idea ?
CDT already has similar Activity support for Terminal command. Thanks @iloveeclipse for the idea.
So I have decided to implement fix for this command, But with controlling activity disablement based on CDT debug ui plugin is loaded or not.

Discussion on limitation of the visibility on the eclipse plaform:
eclipse-platform/eclipse.platform.ui#2900

Other approach to completely remove this command from Run Menu and Main toolbar.
#1143