Redesign the generator button

There are 2 different generator buttons which are sometimes not intuitive to be used.

• Disable them when not the main editor have the focus (e.g. properites view).
• Merge the 2 buttons
• In case a dsl model exist and the generator button in pressed in a xma model start the dsl-generator instead of the xma generator. (Improve update situation and solve potential classloader problem)