Also install the importer executable to OpenBoard root install folder

This reverts commit 4aa0ee61.

- "Importer" folder renamed "importer"
- corrected detecting of 64bit architecture
- build context file is created in build.sh

When the time comes to merge dev into master (for real), the
following may be useful:
https://metlos.wordpress.com/2012/01/13/
git-merging-after-a-revert/

Q_WS_MACX -> Q_OS_OSX
Q_WS_MAC -> Q_OS_OSX
Q_WS_WIN -> Q_OS_WIN
Q_WS_X11 -> Q_OS_LINUX