Use config cascade for style.ini files

Use config cascade for style.ini files

This simplifies style loading and allows the plugins like farmer to
extend the the loading of style files. Also adds some first unit tests.

Originally discussed in FS#2703 and #134

Squashed commit of the following:

commit 5ed550f6ba61a1d475b7572cbff18f0d790c78c2
Merge: 0cefed65e cab1833b0
Author: Andreas Gohr <gohr@cosmocode.de>
Date:   Thu Feb 28 13:38:46 2019 +0100

    Merge branch 'styleConfCascade' of https://github.com/annda/dokuwiki into annda-styleConfCascade

    * 'styleConfCascade' of https://github.com/annda/dokuwiki:
      Clean up StyleUtils
      Update StyleUtils test
      Fix array to boolean conversion
      Use better placeholder string in config cascade
      Calls to StyleUtils use the new constructor
      Add test for StyleUtils
      Refactor StyleUtils
      WIP add temporary test comparing two implementations of StyleUtils
      WIP add new style config cascade side by side with the old one

commit cab1833b09a59331a5580fe0e3e6f36d743dea09
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Thu Feb 21 17:54:27 2019 +0100

    Clean up StyleUtils

commit 766e3907cf7b234c3f2fa7bf86f205c1c8c7c6e1
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Thu Feb 21 17:51:15 2019 +0100

    Update StyleUtils test

commit f5c08787ddf9db60e3429421e5c627d5c81d876a
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Thu Feb 21 12:51:53 2019 +0100

    Fix array to boolean conversion

commit c9671698992328d5671a43b2707ffc6c5b117423
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Thu Feb 21 12:38:55 2019 +0100

    Use better placeholder string in config cascade

commit 4845eff54d227325476bb00b7c8402ce58cf72df
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Thu Feb 21 12:24:16 2019 +0100

    Calls to StyleUtils use the new constructor

commit 7a6337a8f8f2cc0cedbf54ae0b43e94033192cb8
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Thu Feb 21 12:22:44 2019 +0100

    Add test for StyleUtils

commit 5ae9e8f69e076ab2a18b24aa178cca15fea6b000
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Thu Feb 21 12:16:57 2019 +0100

    Refactor StyleUtils

commit a2adcc9f37be93f4bcf90fdba4089ea4b0d6436e
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Sat Feb 16 23:58:04 2019 +0100

    WIP add temporary test comparing two implementations of StyleUtils

commit 845c40c4faf379a65134e171514a09d47ffe95f1
Author: Anna Dabrowska <dabrowska@cosmocode.de>
Date:   Sat Feb 16 23:59:41 2019 +0100

    WIP add new style config cascade side by side with the old one