add TEMPLATE_DOKUWIKI_PAGETOOLFLOAT_DISPLAY event to template

This adds a custom event to the 'dokuwiki' template that allows plugin
authors to easily integrate custom pagetool buttons into the template.
rename event to TEMPLATE_DOKUWIKI_PAGETOOLS_DISPLAY

also made it more secure against copy'n'paste
Merge pull request #236 from splitbrain/pagetoolsevent

add TEMPLATE_DOKUWIKI_PAGETOOLFLOAT_DISPLAY event to template