help show that menu labels don't normally need to be edited &
display the mnemonic correctly). Clicking shows an input dialog to
set a new menu item label.
Split dialogs_show_input() into 2 functions: one simple, one for
a persistent dialog.
Fix possible double-destroy of input dialog when closed by user.



git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@4754 ea778897-0a13-0410-b9d1-a72fbfd435f5