gschem: Fix multi-attribute editor lockup when right-clicking during edit.

This fixes a lockup when right-clicking on the text-edit box for the
attribute value column during an edit. The context menu being popped
up would cause our cell to loose focus, and the editing to be cancelled.

In general, it feels nicer to leave the edit open until the user explicitly
cancels it anyway, and that fixes this issue - whilst departing from the
standard GTK behaviour. (Which is to cancel a cell edit if the edit box
looses focus, but, WITH special case code to handle the context menu case).

A followup patch will be required to add consistent focus / edit cancelling
behaviour on the name column, which uses the standard GTK widgets for editing.
Revert "Move selection monitoring out of the multi attrib widget."

This reverts commit 1aaf6613a7c6991d858c1143ed3917675a115a5a.

This is the first step in implementing attribute editing multiple for
multiple objects at once. The SELECTION object is actually a smart list
of OBJECTs, with list-change notification. Passing this to the dialog
is NOT a break of abstraction, no more than passing a GList of OBJECTs
would be.

Think of the "selection" property of the multi-attribute editor as a
variable list of objects for it to edit, and there is no reason for
separating out this code.
x_multiattrib.c: Pass the main multiattrib class to internal functions

This allows them to access the members they need internally, rather than
having to pass the relevant data in as parameters.
x_multiattrib.[ch]: Whitespace changes for better code consistency
x_multiattrib.c: Rename the "selection" property to "objects"

This is brings us closer in spirit to what the reverted commit
1aaf6613a7c6991d858c1143ed3917675a115a5a was intending to achieve.
Allow editing attributes of multiple objects at once
Update the NEWS file to reflect the multi-attribute editor changes.
gschem: Additional factoring out GschemPageView class
gschem: Add damage rect functionality to GschemPageView
gschem: Remove references to page_current from x_preview.c
gschem: Remove references to page_current from x_event.c
gschem: fix typo

Signed-off-by: Edward Hennessy <ehennes@sbcglobal.net>
gschem: Add setter to GschemPageView for the current PAGE
gschem: Move more scrollbar functionality into GschemPageView
gschem: Remove unused functions
gschem: Remove scrollbar variables from GschemToplevel
gschem: Fix memory leaks
gschem: Factor out calls to SCREENtoWORLD
gschem: Remove superfluous code
gschem: Factor out calls to SCREENabs, SCREENtoWORLD, and WORLDtoSCREEN