[Modal] Return control over useflex again, but provide unsupported debug warning

[lubber-de]

Description

This PR gives full control over the useFlex setting again.
Some combinations are not working for correct positioning, which was basically fixed by #1079 and #1096
However, the fixes ignored a possible explicit given useFlex:true and always assumes auto instead (which makes sure the positioning will work all the time)
As it turned out here there are some usecases, where people need the explicit setting of useFlex:true, i reverted the logic for the can.useFlex method but added some debug warnings instead when unsupported combinations occur, because the positioning for useFlex:true will not work when:

• IE11 is used
• detachable:false is used

As the default of useFlex is auto, existing code should basically work as before without any visual issues

I am totally fine if @fomantic/maintainers will disagree and would like to reject this PR and keep the current implementation because

• there won't be visual positioning issues when using any of the above combinations
• Users would need to enable the debug setting of their modals in order to see the message telling them why the positioning seems to be broken

If we reject this PR and keep the source as it is, we might change the docs, because useFlex:true will definately not work as intended and is currently always treated as auto

🗯 Let me know about your opinion

Testcase

Scroll down to see the button to open the moda
http://jsfiddle.net/9cf2o75b/

Screenshots

When useFlex:true is used in combination with

• IE
  

• detachable: false
  

[exoego]

I am OK with this PR.
I think this warning help developers who need to consider IE11, and the code is trivial enough to be maintained.

[prudho]

LGTM