Fix reloading documents when they are changed fast, multiple times.

This can for example happen when doing interactive git rebases.

* When a file was changed after we reloaded it, but before we removed
+readded the watcher, we wouldn't get a notification for these changes.

* When we got a file changed notification while we were in checkForReload
(because of event processing of the dialog, or file loading), the
notification was dropped and only processes when a check was triggered
again (e.g. at window activation, or when some (other) file changed)

Change-Id: Iab1861a8f05c739a3405bc1afe90ae6f2145057b
Task-number: QTCREATORBUG-9745
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>