Explorer and open editors should use enablement via menu service

[isidorn]

In the current stable vscode we use enablement state to show if an action is applicable on the current context in the open editors and explorer menu.
In the current insiders we simply do not show the actions which are not applicabe since we have adopted the open editors and explorer to use the menu service. And we use the when clause.

We should look into if it is possible for the MenuService to expose an enablement clause.

@jrieken is this already exposed? I could not find it
fyi @bpasero

[jrieken]

The idea is that a command has a precondition which is must be fulfilled in order to run. Editor commands already have that. An example is that the format action cannot run a readonly-document.

For commands, I did add ICommand#precondition but the enforcement must happen in the right context (not how it's currently done). In essence, the user-gesture must decide how to handle the precondition, e.g. a menu item shows it as disabled, and it must do that in the same context its uses for visibility.

[jrieken]

@isidorn scrap this comment... I undone ICommand#precondition and I have added ICommandAction#precondition and adopted it in MenuItemAction. Now, when registering a command you can define its precondition and when calling Menu#getActions the returned actions will have the state set properly.

[isidorn]

@jrieken thanks for this, it works great
@bpasero I compared with the current stable and added some preconditions (for save, compare with saved, revert paste). I had to introduce a dirtyEditor context for this.
Let me know if you have more precondition ideas.