texttemplate: Add public `regexp` property to make this module flexible

It would be nice to have a public regexp property in the texttemplate module, that's allow to specify any other matches.

Right now, the texttemplate module supports only hard-coded template matches in a form like ${foo}. But this is useless when, e.g. we need to porting some CMake project to Qbs project.

E..g. in CMake is useda following form @foo@, so, currently it it impossible to use the texttemplate module in this case.

This module right now used the hard-coded regexp:

var rex = /\${(\$|\w+)}/g;

which can't be changed at all.

So, I suggest to make this regex a public, like:

Module {
    property var regex:  /\${(\$|\w+)}/g
}

which can be changed e.g. with:

\@(\w+)\@

or any other.