Project source tree is a same for all build configurations so it is
a good idea to keep it persistent between CMake runs, configurations
switches and so on. It safes a lot of time for big projects.

Move more operations to the scanner thread:
 - Nodes filtering: skip .user files on top level of the project, skip
well-known extensions and octet-streams: In most cases these are not
required to be shown in the project tree.
 - Nodes sorting

Fix small memory leak: we have .user in the scanner result. After this
node filtered out, but is not feed (old code at the
BuildDirManager::generateProjectTree()). Now .user file skips during scan
without memory allocation at all.

Allow user manually rescan project tree by call Build -> Rescan project
tree. It runs CMake and Tree Scanner together: in most cases only CMake
run requires but time to time (VCS update) full rescan also required.

Change-Id: I4a6e6c897202da557509291c79932dd7751860e5
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>