Tests: Add preliminary IEC-61131 language test, to be continued.

Test sets BEREMIZ_TEST_CYCLES in order to speed-up TON duration test.

Tests: Add time emulation feature for tests with BEREMIZ_TEST_CYCLES CFLAG.

Adding BEREMIZ_TEST_CYCLES=1000 in a project's CFLAGS will:
- run 1000 cycles with no pause
- emulate time flowing normaly for PLC code
- exit PLC thread

This allows:
- testing standard library blocks that deal with time without having to wait
- unit testing and code coverage with POUs that uses time

IDE: fix exception when re-opening previously closed tab

Same as commit with same message, this time for tabs that are not POUs editors.

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.