Add user data to GeanyInputCallback, avoiding global variable hacks
dialogs: Remove an unnecessary function indirection

Having add_input_widgets() didn't really make the code any simpler,
rather obfuscating it a little.
dialogs: Don't abuse GObject data
Merge branch 'small-dialogs-cleanup'
about: Don't leak global variables to the program scope
Remove an unnecessary dynamic allocation
Remove dead assignment

`entries_modified` global is only used by on_name_entry_changed() and
on_entries_changed(), both of which are only ever called from
project_new() scope -- which already initializes this variable.
project: Sanitize entries_modified scope
Fix relative project base path when creating a new project

Closes #1062.