This commit adds a simple main menu to the application. For the time being
it only has 2 submenus, "File" and "Visualizations". The latter is created
by MdiDockArea and lets users manage the visibility and placement of
canvas windows.

reverted changes from e7b11bfa: "added s_changed signal of properties also being propagated to its shared properties"
This was weak design and lead to problems with the AutoEvaluationPipeline.

Introducing usage of C++11 lambda functions instead of functors in some tbb::parallel_for blocks for better readibility

Merge branch 'log-highlighter-double-free-fix' of /mnt/bigone/git/repositories/berge/campvis into development

In its destructor, LogHighlighter first of all deleted its filter regex
explicitly, and then called `setFilterRegExp(0)`, triggering a second —
this time somewhat implicit — delete. If the filter regex pointer was
not NULL, this caused the application to crash due to a double-free.

Fix this error by not invoking `setFilterRegExp` in LogHighlighter's
destructor; the explicit delete performs the same cleanup as
`setFilterRegExp` anyways.

 * fixed 1D geometry TF editor
 * introduced OptionProperty to specify gradient computation mode
 * added s_changed signal of properties also being propagated to its shared properties
 * fixed sampling rate compensation when using adaptive sampling in simpleraycaster.frag

Fixed missing/wrong deregistration of GeometryXdTransferFcuntionEditors when their widgets are closed (and hence the GL contexts destroyed)

Merge branch 'fix-const' of /mnt/bigone/git/repositories/sebastian.poelsterl/campvis into development

Merge branch 'revert-no-tree-vrp' of /mnt/bigone/git/repositories/sebastian.poelsterl/campvis into development

    This reverts commit 442d5051.

Fixes -Wignored-qualifiers

Added -fno-tree-vrp to CMake definition, since tree-vrp optimization breaks the code for some strange reason.

Fixes -Wsign-compare

Fixes -Wignored-qualifiers

Fixes -Wswitch

Fixes -Wreorder

Previously, QtCanvas' init() would call show() on its underlying widget
and cause it to appear as a top-level window for a split second, before
the canvas was docked. To avoid this flicker, show() is now only called
on the canvas after it's been added to the MDI area.

Previously, when an MDI window was being dragged very fast and its state
changed from docked to floating (or vice versa), it wouldn't be placed
under the cursor, but instead some way behind it (with regards to the
drag direction). This happened because we decided where to place the
window's based on its last position in its previous state. Now we use
the cursor's position to calculate the window's new position, which
fixes the problem.

PipelineMdiSubWindow has been renamed MdiDockedWindow, and
refactored to make it easier to use it with arbitrary widgets.

To better group MDI-related classes, all of them have been moved to the
´gui/mdi´ directory.

VisualizationPipelineWidget has been renamed MdiFloatingWindow, and
refactored to make it easier to use it with arbitrary widgets.

VisualizationPipelineWrapper has been renamed MdiDockArea, and refactored
to make it easier to use it with arbitrary widgets. It now inherits from
QMdiArea, which removes an unnecessary layer of indirection. Moreover, it
creates MDI subwindows and floating windows only if necessary, i.e. when
a widget stored in it changes state.