Summary of work done in commit:
- Necessary Workbench DLLs for startup have been added to Mantid's
deployment
- launch_workbench startup Python script has been added, with supporting Batch and PowerShell wrapper scripts to use the correct Python from Mantid's Installation
- The PowerShell startup script is compiled into an EXE during CMake
Generation, this is done as a step in WindowsNSIS if ENABLE_WORKBENCH
and PACKAGE_WORKBENCH are set to true, and PowerShell is available. The
executable is compiled for PowerShell 2.0 and it should be possible to
run it on every Windows 7 and up.
- Workbench statup scripts installation has been moved to WindowsNSIS
- Added placeholder icons for Workbench, used only in the Windows
installation

Original commit messages:

Added launch_workbench startup script and batch file, re #23263

Added missing DLL and launch_workbench executable, re #23263

Added launch_workbench startup scripts, re #23263

Fixed getting the launch_workbench script's directory, re #23263

Added check for PowerShell, and workbench image holder, re #23263

Polished workbench batch script and Windows CMAKE file, re #23263

Added workbench unstable and nightly placeholder icons, re #23263

Moved workbench Windows install calls to WindowsNSIS, re #23263

PS will launch the error reporter when ExitCode != 0, re #23263

The Workbench PowerShell run script will now check the ExitCode of the
Workbench process, and run the Error dialog if the ExitCode is not 0

CMake added whitespace on if statements, re #23263

The former purely contained icons for the instrument view. These
now live in a separate resource file compiled with the library
rather than with the MantidPlot executable so that they are
always available wherever the library is used.

Added PNG icons for nightly and unstable, used by NSIS for the package icon

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This is mainly an issue with hi res screens and 200% scaling

re #20607

re #19896

ILL logo remains though

re #19897

Also add the layered image file (Paint dot net) to allow easier editing in the future

re #19896