Add ctkCollapsibleButton::flat property

Closes #502
Merge branch '502-flat-ctkCollapsibleButton'

* 502-flat-ctkCollapsibleButton:
  Add ctkCollapsibleButton::flat property
ENH: Clear box icon replaced with a less harsh one, a simple 'x' that does not indicate error
Merge pull request #503 from cpinter/501-search-box-clear-icon

Clear box icon replaced with a less harsh one, a simple 'x' that do...
Add missing error log context parameter.
Fixed Qt4/5 MOC issues regarding QIconEngineV2.
Fixed incompatibilities between Windows.h and Qt 5 (NOMINMAX).
Add NOMINMAX definition to global compile flags.
Change sleep duration from 200ms to 250ms.
ctkXnatResource download function implemented
Set QT_INSTALLED_LIBRARY_DIR in case of Qt5.
Merge pull request #505 from kislinsk/504-qt5-fixes

Make CTK compatible with Qt5 again.
Replaced Qt5::Gui dependency with Qt5::Widgets.
Merge pull request #511 from kislinsk/command-line-modules-require-qt5-widgets

Replaced Qt5::Gui dependency of CommandLineModules/Core with Qt5::Widgets.
BUG: Fixed missing list elements problem

When there are two strings with the only difference that one is longer (e.g., "Label Statistics" and "Label Statistics (BRAINS)") then one of them is missing from the search results.

The problem was that QAbstractItemModel::match was used for checking if the string is already in the model and by default Match uses Qt::MatchStartsWith option.

Fixed by using Qt::MatchFixedString.
Merge pull request #512 from lassoan/fix_menu_combobox_search_missing_item

BUG: Fixed missing list elements problem
Merge pull request #506 from tomdoel/xnat-resource-download-fix

ctkXnatResource download function implemented
Added an option to skip install commands for plugins.
Using : as separator might conflict with absolute paths on Windows.
Ensure CTKVisualizationVTKWidget build with VTK6 and python disabled.