Update CKEditor auf Version 4.15.0

[Torsten Riemer]

## CKEditor 4.15

New features:

• #3940(​https://github.com/ckeditor/ckeditor4/issues/3940): Introduced the colorName property for customizing foreground and background styles in the [Color Button](​https://ckeditor.com/cke4/addon/colorbutton) plugin via the [config.colorButton_foreStyle](​https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html#cfg-colorButton_foreStyle) and [config.colorButton_backStyle](​https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html#cfg-colorButton_backStyle) configuration options.
• #3793(​https://github.com/ckeditor/ckeditor4/issues/3793): Introduced the [Editor Placeholder](​https://ckeditor.com/cke4/addon/editorplaceholder) plugin.
• #1795(​https://github.com/ckeditor/ckeditor4/issues/1795): The colors picked from the [Color Dialog](​https://ckeditor.com/cke4/addon/colordialog) are now stored in the [Color Button](​https://ckeditor.com/cke4/addon/colorbutton) palette and can be reused easily.
• #3783(​https://github.com/ckeditor/ckeditor4/issues/3783): The colors used in the document are now displayed as a part of the [Color Button](​https://ckeditor.com/cke4/addon/colorbutton) palette.

Fixed Issues:

• #4060(​https://github.com/ckeditor/ckeditor4/issues/4060): Fixed: The content inside a [widget](​https://ckeditor.com/cke4/addon/widget) nested editable is escaped twice.
• #4183(​https://github.com/ckeditor/ckeditor4/issues/4183): [Safari] Fixed: Incorrect image dimensions when using the [Easy Image](​https://ckeditor.com/cke4/addon/easyimage) plugin alongside the [IFrame Editing Area](​https://ckeditor.com/cke4/addon/wysiwygarea) plugin.
• #3693(​https://github.com/ckeditor/ckeditor4/issues/3693): Fixed: Incorrect default values for several [Color Button](​https://ckeditor.com/cke4/addon/colorbutton) configuration variables in the API documentation.
• #3795(​https://github.com/ckeditor/ckeditor4/issues/3795): Fixed: Setting the [config.dataIndentationChars](​https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html#cfg-dataIndentationChars) configuration option to an empty string is ignored and replaced by a tab (\t) character. Thanks to [Thomas Grinderslev](​https://github.com/Znegl)!
• #4107(​https://github.com/ckeditor/ckeditor4/issues/4107): Fixed: Multiple [Autocomplete](​https://ckeditor.com/cke4/addon/autocomplete) instances cause keyboard navigation issues.
• #4041(​https://github.com/ckeditor/ckeditor4/issues/4041): Fixed: The[selection.scrollIntoView](​https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_dom_selection.html#method-scrollIntoView) method throws an error when the editor selection is not set.
• #3361(​https://github.com/ckeditor/ckeditor4/issues/3361): Fixed: Loading multiple [custom editor configurations](​https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html#cfg-customConfig) is prone to a race condition between these.
• #4007(​https://github.com/ckeditor/ckeditor4/issues/4007): Fixed: Screen readers do not announce the [Rich Combo](​https://ckeditor.com/cke4/addon/richcombo) plugin is collapsed or expanded.
• #4141(​https://github.com/ckeditor/ckeditor4/issues/4141): Fixed: The styles are incorrectly applied when there is a <select> element inside the editor.

[Torsten Riemer]

In 12925:

fix #1913 - update CKEditor to version 4.15.0

[Torsten Riemer]

In 12926:

fix #1913 - update CKEditor to version 4.15.0 - removed trial version of plugin pdfexport using CKEditor Online Builder