removed insert text, paste, and mouse release handler functions

it is no longer necessary to have the insert text function because are
text insertions are now caught by the document changed slot function

the purpose of the reimplemented paste function was to catch inserted
text from pasting using the paste action (menu, context menu and tool
bar), but now the document changed slot catches these so it was removed

the paste key sequence was added to intercept pasting of text from the
key command, but this is also no longer necessary

the purpose of the reimplemented mouse release event handler was to
catch inserted text from middle-click pasting, so it was also removed

the paste selection function was used by both mouse release event
handler (no longer needed) and for allowing this type of pasting from
the shift+control+insert key sequence (still needed) can now call the
edit box's text cursor insert function to insert the selection text