Reconcile mkforms and rn_base 1.14

[oliverklee]

rn_base 1.14.0 breaks the mkforms extension. I will take care of fixing this and will greatly appreciate your help in the form of reviews, merges and releases for this.

• rn_base: use the namespaced base test case
• rn_base: add a class alias for Tx_Rnbase_Frontend_Plugin
• rn_base: move the base test case out of tests/ into Classes/ (and provide class aliases for this)
• rn_base: finish streamlining the class aliases
• rn_base: release another version 1.14.x
• mkforms: add PHPStan (this will allow us to more easily catch usages of removed classes and incompatible API use)
• mkforms: use AbstractPlugin instead of Tx_Rnbase_Frontend_Plugin
• mkforms: use the new BaseTestCase
• mkforms: require rn_base ~1.13.15
• mkforms: replace usages of rn_base classes that are deprecated in rn_base 1.13.15
• mkforms: as a test, allow installations with rn_base 1.14 again and find out all remaining issues
• mkforms: fix remaining issues with rn_base 1.14
• mkforms: allow installations with rn_base 1.14 again
• mkforms: release another version 1.10.x
• mkforms: require rn_base >= 1.14
• mkforms: replace usages of rn_base classes that are deprecated in rn_base 1.14

This tracking bug has a cousin on the side of the other extension.