CMake: Add ability to batch edit user defined variables

In Qt Creator 4.3 one can add a user defined variable by hand, one by one. This can become easily tedious after a few variables.

After a variable has been added, it will be displayed with a bold font. 

Unfortunately if one opens the settings dialog after a reboot the manually added variables are not displayed with bold anymore!

A big project can have tens of CMake variables, which makes it very hard to see which variables were user defined or not, even with bold fonts.

Qt Creator should provide the same functionality it provides for Environment Variables, which get a different view for user defined variables and they can also be edited in a batch mode.

Batch edit makes it very easy to copy settings from project to project.

When Qt Creator adds a user defined variable this ends in the CMakeCache.txt like this:

//No help, variable specified on the command line.
CMAKE_USE_CCACHE:BOOL=ON

Qt Creator doesn't add a comment, it could use "Qt Creator manually added variable" and based on this to display them in the Project settings separately.

The CMakeCache.txt variable style can be also used in a batch edit mode! Just specify the type after the colon!