Allow module installation if ckeditor plugin has not yet been installed

CKEditor Indentblock cannot be installed, if the CKEditor Indent Block plugin has not been installed before. That interferes with possible command line support add-ons like a drush add-on for downloading the Indent Block plugin included in the module, because this needs the module already to be enabled.

To add this feature the CKEditor Indent Block module shall:

1. Only set a warning in hook_requirement during install and runtime phases to allow the module to install.
2. Set a warning when the module is installed without the CKEditor plugin installed.
3. Set a warning when on "Text formats and editors" configuration of text format, if the plugin is not installed.
4. Disable the Indent Plugin "Enable indentation on paragraphs" checkbox, if the plugin is not installed.