Prevent reentrant calling of ctkProxyStyle::ensureBaseStyle()

If ctkProxyStyle::ensureBaseStyle() is already in progress for an object then further ctkProxyStyle::ensureBaseStyle() calls for the same object are ignored.
This patch is tested and does prevent the crashes. There doesn't seem to be any side-effect.
Merge pull request #372 from lassoan/371-ensure-base-style-infinite-recursion-crash

Prevent reentrant calling of ctkProxyStyle::ensureBaseStyle()
Added check to only grab visible child widgets

Trying to grab a non-visible widget may lead to an application hang, because glGetError may never clear the error if a GL context is not created.
See details at http://www.na-mic.org/Bug/view.php?id=3438
Merge pull request #374 from lassoan/373-grabVTKWidget-hangs

373 - Added check to only grab visible child widgets
Fix indent style for braces in if statement
Merge branch 'master' of github.com:cpinter/CTK into 276-dicom-database-display-tables
COMP: Fixed merge errors after being able to build CTK on Windows
COMP: Disabled default paths for DCMTK build