Add a configurable search button

Hi,
now I added the configurable search button. 
Default will be off.
If added searchbutton=true, it will be displayed.

Kind regards
Cornelius

In some circumstances (browser, website) using enter to trigger the search leads to
side effects. So having a button to trigger the search is easier.

The searchbutton can be activated with
  searchbutton : true

The text of the searchbutton can be set with
  searchtext : "Search"