Prompt to overwrite in Windows Save As dialog

Add OFN_OVERWRITEPROMPT so the dialog will prompt before overwriting an existing file.
Remove the OFN_FILEMUSTEXIST flag, which works only with Open dialogs.
Merge branch 'kugel/document-messages2'

This is an extension of the long-lived `document-messages` branch.

It is mostly adjusted according to the feedback on the mailing list.
There might existing some usability issues to be resolved, but this
is the first step towards non-modal document notifications.
notebook: Fix crash on tab close

Since infobars notebook_tab_close_clicked_cb() wants a GeanyDocument as userdata.
show_tab_bar_popup_menu() on the other hand wants the actual GtkNotebook child.
Don't use the notebook page widget for tab popup

Directly use the GeanyDocument rather than pass the notebook page
widget and get the document from that.  This makes the code more future
proof and less weird.
Remove unused variables and avoid variable shadowing
Merge PR#113
Remove empty table rows in the Glade file

This avoids glade from emitting some useless placeholders.
No UI changes.
Bump min. version of GTK2 to 2.24 and GLib to 2.28

Note: This is mostly search and replace. I've only tested with my
GTK+ 2.24.20 and not at all the Waf build system.
Remove pre-GTK+ 2.24 preprocessor checks and related code