Code refresh + SPM support

[Climbatize]

This PR is used in production since the last 9 month with no issues so far. (through this SPM dependency: https://github.com/Climbatize/XLPagerTabStrip.git > 9.1.0)

• Updated source code to remove warnings and use clang instead of GCC.
• Upgraded support to iOS 11 and added support for SPM.
• Moved FXPageControl to a SPM dependency
• Fixed deprecated code concerning window orientation.

I know there is already this good PR https://github.com/xmartlabs/XLPagerTabStrip/pull/846 aiming to bring support to SPM, but I think this lib needed a deeper refresh to remove some deprecations.

[Climbatize]

Thanks @mats-claassen for the quick and kind review! I left my comments and provided update based on your feedbacks.

[mats-claassen]

@Climbatize changes look good. Just the matter of the weak outlets remaining

[Climbatize]

I reverted changes on weak outlets. At first sight no crashes from my side. I think we're good to merge @mats-claassen :)

[Climbatize]

Thanks for the merge @mats-claassen ! Don't forget to tag the master branch to a minor version greater than 9.0.0 so the release is accessible, or to edit the the readme file to tell devs to point to master branch