Update/proofread Kate handbook

kate is by kde, not for kde
move all plugin related info to plugin.docbook
remove all obsolete infos for kate plugins
add a lot of FIXME's for missing content, will be updated in next RR
moved every plugin infos (menu + config) into the corresponding plugin section, if there is any
new/updated screenshots
pngs for icon button repacing labeled buttons
REVIEW:127949