Welcome dialog

[actionless]

"oh instead of doing real help they just put a link to 100 pages of wiki, so i'll just close it instead of reading'

Even though I pointed to Builder as an example, but in Themix, I think having only several sub-pages on top page of wiki is enough.

Anyway I share my idea of a startup/help dialog something like this:

                    Welcome to Themix                                <-- title

  Are you new to Themix? Online documentation is available!

  [ How to create and export theme ]                                 <-- link button
  [ How to import and export base16 ]                                <-- link button
  [ How to create themes from images ]                               <-- link button

  [x] Don't show show this dialog at startup.                        <-- checkbox

----------------------------------------------------------
                            OK                                       <-- button

Originally posted by @nana-4 in #186 (comment)