Prevent JobManager UI freeze when waiting for a rule in the UI Thread

[mickaelistria]

This is a followup of discussions in #6 and former Gerrit patch.

The general problem is that when one wants to run a job or some other operation protected by scheduling rules in UI Thread, and this scheduling rule is currently "blocked" by some other work, then the result is a UI Freeze.
We could do better by allowing the workbench/IDE parts to customize a bit the JobManager, more specifically the beginRule so it would show the interesting ProgressDialog when called from the UI Thread, so it doesn't seem too much like a freeze, and users have the opportunity to workaround it. It would also ease debugging.
Such a solution would allow to serve multiple operations that have implemented workarounds for this freeze, such as #6 , and those workarounds could then be removed.

[laeubi]

I don't think we need more JobManager Customizations here, the underlying problem is more that in some circumstances the UI-Events are not driven, see

• UI freezes when a ThreadJob is waiting to run in the UI Thread eclipse.platform.runtime#40

for an analysis and fix.What is left is to just need to override these new methods .

[mickaelistria]

#40 is promising, I suggest we follow that track first and then we'll evaluate if we needs further smartness here and there.