This should be faster than copying SourceFormatterStyle at each call of
this function.

Introduce a QString styleContent variable in
CustomScriptPlugin::formatSourceWithStyle() for these reasons:
* style is now a reference to const and cannot be assigned to;
* the name of an incomplete style matches the name of the corresponding
  predefined style, so using unmodified style.name() below is correct;
* QString content() is the only other member of style used below.

Reformat the lines touched by this change.

Don't refer to the previous name of kdevcustomscript plugin "indent" in
the warning message touched by this change.