Allow specifying the data type of each column during import manually
Description
Allow specifying the data type of each column during import manually. This is how it looks in Libreoffice Calc:
Labplot Version
Debug build
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-9u55M9/labplot-2.10.1+git202308181832~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