Editor: only enable outdent/indent when appropriate

[liucougar]

should not allow user to indent neither a non-list node nor list item which is the first item in its parent

should not allow user to outdent a non-list node

[bill]

This was changed in [13262].

Change was for the following reasons (according to Cougar):

• On one or more browsers, if you indent a block of text you can't get it to unindent.
• works differently on different browsers. some use <blockquote>, others adjust the paragraphs padding/margin.

Note also that adding a <blockquote> to the list of formatting options (along with <p>, <h1>, <h2>, <ul>, etc.) is problematic because there's apparently no way to convert a <blockquote> to a <p>. Since a <blockquote> can contain multiple paragraphs, selecting <p> just inserts a paragraph into the blockquote.

[bill]

Note: rolled back in [18437], after normalizing every browser to use <blockquote> in [18417].