IDE: fix again ruberband with gtk3.

DC logical functions are now disabled when using GTK3.
Apparently using XOR was still having an effect.
Use regular black pen with no logical funciton instead.

Merged from default

merged

Modbus web configuration: add check for valid delay and period parameters

IDE: Fix missing highlighted element in diagrams when using GTK3

- wx's logical function does not work on gtk3, and highlighting relies on it
- skipping logical funciion at all "works" but this now makes highlight opaque
- no easy fix for highlight transparency since emulated with AND Wx logical function
+ added other highlight related cosmetic fix.

Merged default in wxPython4 branch

Linux runtime: overrun detection for real-time timers and for plc execution.

If real-time timer wakes-up PLC thread too late (10% over period), then
warning is logged.

If PLC code (IO retreive, execution, IO publish) takes longer than requested
PLC execution cycle, then warning is logged, and CPU hoogging is mitigated
by delaying next PLC execution a few cylces more until having at least
1ms minimal idle time.

Fix typo in toolchain_gcc.py : LDLAGS -> LDFLAGS

Merged from default

Merge.

SVGHMI: add two more simple widgets to default widget library

Cosmetic fix in plc_debug.c template

Merge.

Modbus plugin: add optional delay between two consecutive modbus requests sent by master/client

Runtime: Add RealTime checkbox in Linux target options to enable PREEMPT_RT scheduling.