Warn user on attempting to switch to a workspace that is currently
locked by another Eclipse instance. Error dialog is now shown informing
user about the lock details and preventing Eclipse from exiting.

- Refactored code to use common WorkspaceLock class
- Moved related messages to the workbench bundle
- cleaned up documentation and code after
be51d14c6f825ce5f072dcbf771449620e789b6a changes.

Also-by: Andrey Loskutov <loskutov@gmx.de>

See https://github.com/eclipse-platform/eclipse.platform.ui/pull/3518