Add an option to run a selected CAS engine at LabPlot startup

Description

Add an option to run a selected CAS engine at LabPlot startup. I would like to see Qalculate! run by default as an advanced calculator in LabPlot. However, other engines could also be set to start at LabPlot startup.

See also this issue: Add the Qalculate! library by default to Windows builds #695

Labplot Version

System: Ubuntu 23.04
Locale: English,United States (Decimal point ',', Group separator ' '
Number settings: Decimal point ',', Group separator ' ', Exponential 'e', Zero digit '0', Percent '%', Positive/Negative sign '+'/'-' (Updated on restart)
Architecture: x86_64-little_endian-lp64
Kernel: linux 6.2.0-27-generic
C++ Compiler: GNU 12.3.0
C++ Compiler Flags: -g -O2 -fdebug-prefix-map=/build/labplot-GsQbHV/labplot-2.10.1+git202308251632~ubuntu23.04.1=. -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fno-operator-names -fno-exceptions -Wall -Wextra -Wcast-align -Wchar-subscripts -Wformat-security -Wno-long-long -Wpointer-arith -Wundef -Wnon-virtual-dtor -Woverloaded-virtual -Werror=return-type -Werror=init-self -Wvla -Wdate-time -Wsuggest-override -Wlogical-op -Wall -Wextra -Wundef -Wpointer-arith -Wunreachable-code -Wunused -Wdeprecated-declarations -fno-omit-frame-pointer -fstack-protector -fexceptions -std=c++17 -O2 -Wcast-align -Wswitch-enum -fvisibility=default -pedantic -Wzero-as-null-pointer-constant