checkfunc is mainly replaced by doku_hasTextBeenModified, which is put
in  global scope to allow overriding by plugins.

In php7 constructors with the same name as the class are deprecated. __construct() should be used.

Depending on the server-configuration this could result in a warning which would break the API.

If using a west handle, jQuery UI Resizable uses left offset to
position, assuming relative:absolute like setup.

- http://stackoverflow.com/questions/28512111/resizable-div-that-
doesnt-change-left-property
- https://bugs.jqueryui.com/ticket/4985#comment:1

- if #wiki__text (the default DokuWiki editor field) exists, the
checkfunc() checks whether the field content is changed
- if #wiki__text not exists it assumes that on all change and keydown
events the content of the form #dw__editform is changed (give still
false positives.) (for example the Data plugin has none #wiki__text)

Otherwise you have to reapply the filtering every time you updated one
plugin.

Fixes #1483

Fixes #1199

Prevents overwriting already added stuff.

#1327

See #1551

Changed the values recognized by the general config parser.
If value in config is a boolean it is recognized as boolean, not if it
is a string.

Fixes #718

This drops support for Internet Explorer <9 which we no longer support.

The opera work around is still needed though! O_o

Standardized node.textContent is available since IE9