cxx-qt-lib: use +whole_archive for std_types so that cargo-only works

[ahayzen-kdab]

When building with cargo we need to ensure that the statics aren't optimised out for registering types with Qt 5.

Otherwise we cannot use numbers in QML.

Closes #592

[LeonMatthesKDAB]

I think the wording can be improved a bit, but otherwise looks good 👍

[LeonMatthesKDAB]

LGTM

[Be-ing]

I'm concerned that compiling the same C++ code unconditionally with cxx-qt-build could lead to duplicate symbol errors if a user tries to have multiple crates in one project use cxx-qt. If the C++ code is moved to the cxx-qt or cxx-qt-lib crates and multiple crates in the dependency graph use cxx-qt, IIUC they would all share the same build of the C++ code that would only need to be compiled once. But if multiple crates run cxx-qt-build in their build.rs, then those crates are linked into one staticlib (like Firefox does #148), this might cause duplicate symbol errors.

[ahayzen-kdab]

Note that i tried to create a multi crate project, but this doesn't work as you run into

error: link modifiers combination `+bundle,+whole-archive` is unstable when generating rlibs

I tried to have a project where i had the following crate structure each with their own QObject and cxx-qt-build, and then main using public methods from sub1 and sub2.

main (staticlib)
  - sub1 (rlib)
  - sub2 (rlib)

[Be-ing]

But if multiple crates run cxx-qt-build in their build.rs, then those crates are linked into one staticlib (like Firefox does #148), this might cause duplicate symbol errors.

Considering #598 shows this already doesn't work, there's no regression by merging this.