SVGHMI: default library path, default page when starting a SVGHMI project, error page when no SVG present

SVGHMI: finished multiclient support. Still needs more testing.

SVGHMI: Fixed iterator in session manager. Getting closer to working multiclient, to be continued.

SVGHMI: Fixing last commit's multiclient implementation, in case of watchdog. To be continued, since multiclient still fail...

SVGHMI: Implemented multiserver+multiclient, but only tested with single client and single server for now. To be continued...

SVGHMI: More configuration parameters : network interface, TCP port, URL path and watchdog enabling.

SVGHMI: Added a way to distinguish watchdog-enabled HMI from multi-client HMI in URL.

SVGHMI: Prepare accepting multiple clients on multiple HMI: Use POULibrary new capability to add variables to PLC Configuration. Now heartbeat and HMI tree root are added by the library part of SVGHMI, and not by each instance.

IDE: Allow POU Libraries to inject PLC Configurations globals during build.

SVGHMI: DnD UI: Add hint instructing user to Drag HMI tree items to text field.

SVGHMI: update generated XSLT files

SVGHMI: DnD UI: Fix SVG gen that was dropping widget name and making XSLTExceptions plus some UX enhancements.

removed useless import

SVGHMI: DnD UI: SVG for DnD now generated again based on paths and args entries filled by user.