The LinkMap is the widget that draws correspondence lines between two
panes in our FileDiff views, and provides clickable areas for
performing actions on change blocks. This commit isolates the LinkMap
code into its own class and overrides signals as appropriate, rather
than dealing with all callbacks through FileDiff.

In addition, the new LinkMap widget has improved handling of
non-writable TextViews it is associated with, which should fulfil the
requirements associated with merge mode.

meld/linkmap.py: Add new LinkMap class, with most code moved and
                 adapted from the existing implementation in FileDiff.

meld/filediff.py: Remove LinkMap code and add FileDiff/LinkMap
                  integration bits. We also move to treating the
                  window-wide keymask as a property.

meld/filemerge.py: Remove LinkMap code; the new LinkMap should handle
                   merge mode correctly itself.

data/ui/filediff.ui: Adjust UI file to treat LinkMap as a real widget

meld/util/sourceviewer.py: Add some TextView-specific APIs to our proxy
                           widget for use in LinkMap; these APIs will
                           be used in FileDiff, but are not currently.

meld/ui/catalog.xml: Add Glade support for LinkMap
meld/ui/gladesupport.py: Add Glade support for LinkMap