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.
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.
Remove some empty table rows and columns from the Glade file

Cleanup some useless placeholders, and replace some one-column tables
with vertical boxes.

No UI changes.
Fix an inverted check
Properly mirror document_get_from_page() in document_get_notebook_page()

Now document_get_from_page() supports arbitrary nesting inside the
notebook page, update document_get_notebook_page() to support it also,
by searching up for the direct notebook child rather than assuming it
is the ScintillaWidget's direct parent.
Avoid code duplication in document_get_current()
Fix a few missing/extra includes
infobars: Use wrap labels to avoid cropped infobars

Since the infobar is backed in a GtkPaned that allows "shrink", if the
content is too wide, it's far end will just get cropped.  Also, wrap
labels are nicer to display possibly long texts as it flows naturally.
Do not use backward slashes in geany.pc on Windows

And explicitly convert backward slashes in the global
prefix to forward slashes as tools like pkg-config or
gcc could interpret the backward slashes as escape
sequences.
Fix build with GTK < 2.22 and GLIB < 2.32.

Add these back-compat to gtkcompat.h until the actual GTK version we want to depend on is figured out.

Based on SF #1047, and fixes that.