BUG: Persist 'dont show message' selection when deleting markups

This fix addresses Mantis item #3663. The ctkMessageBox expects a
ButtonRole of AcceptRole. Created a custom button for 2 reasons.
First, it allows us to set the correct role and secondly a "Delete"
and "Delete All" button more clearly aligns with the action being
performed.

From: David Stoup <david.stoup@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23315 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Fix overloaded virtual warnings with VTKv6

Explicitly use base class implementations of transform overloads
operating on 'float' (which cast and call the double overloads, and so
will call the appropriate derived code) in order to avoid hiding the
overloads in the derived class. (Fixes -Woverloaded-virtual warnings.)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23316 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Remove gratuitous virtual

Remove 'virtual' specifier of private classes in source files where said
classes are not and (due to being in a source file) never will be
derived. This avoids needing an explicit (virtual) dtor in order to
avoid a -Wnon-virtual-dtor warning.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23317 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: Fix spelling error in comment

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23318 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Don't stop uninstall on first error

Change how we process scheduled extension uninstall operations to
continue trying to uninstall all extensions, rather than stopping the
process after the first error. The old behavior is a result of operand
order to the '&&' operator, which employs short-circuit evaluation to
skip evaluation of the second argument (the call to uninstall the
extension) if the first argument (the overall result 'accumulator') is
false, and was likely unintentional, due to overlooking the short
circuit evaluation.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23319 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Improvements to extension notification icons

Clean up the SVG for the existing extension notification icons, and
change the error icon from a circle to a box (matches oxygen, and
"frees" the circle for the information icon, such that all three have
different shapes). Add an information icon. It's not in use yet, but
will be used for updates.
STYLE: Remove unused icons

Remove some icon files from the repository that are not used anywhere.
This is in preparation for a major clean-up and refactoring.
ENH: Clean up icon usage

Create new "XSmall" icon size for 16x16 icons, enabling us to provide
separate 16x16 (e.g. menu) and 22x22 (e.g. tool bar) icon sizes. Replace
ugly undo/redo icons with icons from the (public domain) Tango[1] theme.
Replace home icon with its Tango version for better consistency. Create
a script to generate qSlicerBaseQTGUI.qrc, as manually adding as many as
five entries per new icon is unnecessarily awkward. Arrange to use
themed icons when possible.

(1: http://tango.freedesktop.org/Tango_Icon_Library)