Feature "qtwidgets" added.

[Atlas42]

Link with QtWidgets and use QApplication when activated (default) Use QGuiApplication when not.

Replaces #288

I rewrote the patch in a new clean branch, keeping the master for my own projects dependency.

As #[cfg] and cpp! does not work well (if not at all together), I used a C++ predefined symbol to configure the used class.
Not as clean as I would like but I lost too much time already to figure out somthing that works.
Tell me if you have a better solution.

[ogoffart]

This is a breaking change so it will require a major version update.

[Atlas42]

major as in 1.0.0 or as 0.3.0 ?
any idea when that will happen ? not a big fan of keeping custom versions for a long time.

Any idea on implementing this without making it a breaking change ?
I thought about using a "no-qwidgets" feature instead but I find this rather dirty.

[ogoffart]

0.3

Could happen soon.
@ratijas is there any other change you would like to do?

[ratijas]

nah, it's not like I'm actively working with anything in Rust lately. But thanks for asking!

Maybe we could consider finilizing and merging #227 as well?

[ratijas]

Since this PR seems to be replacing #211, how about adding a little note to README just like they did?

[ratijas]

is there any other change you would like to do?

Also, this

Or we can just keep this QObjectBox as deprecated and remove it when we have reason to break semver.

#178

[Atlas42]

README entry added, my PR is almost identical to #211