Fix registration of Qt documentation

Restoring project explorer related information was delayed, including
tool chains, which triggers restoring Qt versions. Qt documentation was
registered in QtSupport's delayedInitialize which comes before
ProjectExplorer's delayedInitialize, so no Qt versions were found to
register documentation for.

Register documentation in QtVersionManager::triggerQtVersionRestore
instead.

Change-Id: Ia36f4d89672f2c651157dee50c3e8996af55598c
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>