Translation libraries

The VS tools currently does not allow a user to use the same translation files for multiple Qt projects in a solution. A workaround is possible ( see #QTVSADDINBUG-1185 ) but not user friendly since:

• It requires each relevant file from each project to be entered manually to one of the projects' additional options.
• The user has to then be careful to always call lupdate on the select project, and not the others.

We could instead have a "translation library" target/project, which contains a set of translation files that are shared between other projects. Calling lupdate on a translation library project collects all relevant source files from all projects in the solution (includable/excludable per-project and/or per-file). On build, a translation library project does not create a library/executable, just calls lrelease, and outputs .qm files to a directory reachable by other projects at runtime.

Perhaps even an option to call rcc to embed the translation files to an .rcc file / source files.