rename files with txx extension to hxx

See merge request !152

BUG: fix issue #1639 Update UserValue flag in QtWidgetParameterList

Closes #1639

See merge request !142

OTB followed since the beginning the ITK convention and use .txx extension for all
template classes. Nevertheless, some development tools do not recognize .txx
file extension. Other tool like GitHub can't do in-browser syntax highlighting for
txx files I think.

The root problem is the use of the txx which should be changed to hxx (or hpp).

In 2011, after an in-depth discussion near April 20, 2011 on the
Insight-Developers mailing list, ITK rename all txx files to hxx (and event
prevent the push of .txx files with a pre-commit hook). It happens is major release v4.

You can find some arguments in the discussion about the change and also in other
projects related to ITK which applied the same modification, see for instance VXL:

https://github.com/vxl/vxl/issues/209

This commit apply now the same modification for OTB.

I understand that it will change some habit for developers and don't bring new
features but I think that in general it is better to stay align with ITK guidelines.

In my opinion, it always facilitate the use of OTB and ITK together if we share
when we can the same code architecture, directory organization, naming
conventions...

ENH: display parameter description as tool tip for the QtWidgetBoolParameter

See merge request !149

Resolve "Warn otbcli user about unused parameters"

See merge request !144

REFAC: include-what-you-use in Modules/Core/

See merge request !138

DOC: Minor fixes in Software Guide

See merge request !140

BUG: fix issue !1639 Update UserValue flag when a parameter is modified in QtWidgetParameterList widget

The QtWidgetParameterList widget should signal that an input parameter as been
modify by the user.Then, it set the UserValue flag from the corresponding parameter to
True.
It was spotted as the UserValue flag is used by the ParseApplication method used
to list parameter modified by the user in the GUI which is used to build the
corresponding command-line from the application GUI (displayed in the Log tab).

COMP: fix gcc 7 'defined' warnings

See merge request orfeotoolbox/otb!127

DOC: Set of proposed edits to the documentation

See merge request orfeotoolbox/otb!137

C++14: Replace comments by '= delete'

See merge request !133

BUG: Fix missing QWidget parents

See merge request !125

Conflicts:
	Modules/Wrappers/QtWidget/include/otbWrapperQtWidgetBoolParameter.h

C++14: Replace ITK_OVERRIDE by override

See merge request orfeotoolbox/otb!131

C++14: Replace ITK_NULLPTR by nullptr

See merge request !130

Update screenshots in Monteverdi Cookbook section with wrong RGB color composition

Closes #1627

See merge request !129

BUG: Remove workaround for QT bug 22829

See merge request !126

Document third party environment variables

See merge request !124

DOC: add checklist to MR template

See merge request !136

New custom widgets for Float and Int parameters

See merge request !123