Rewrite mouse cloning

Adding support for gnome-shell 46 has introduced a new bug:

When cursor cloning is enabled, resizing windows causes the cursor to
freeze up until ESC pressed, after which clicking on windows to change
focus no longer works correctly until the extention is disabled.

There did not appear to be an easy "fix" for this, as there aren't any
obvious differences in how cursors are dealt with in between gnome-shell
45 and 46.

This commit simplifies and refactors mouse cloning, fixing the bug in
the process.

Note that it does not jive with the magnification tool -- it ends up
showing two cursors in slightly different positions.  Adjust the warning
in the preferences window to read as such, unfortunately breaking
translations in po/.

See GitHub issue:
#20

schemas/org.gnome.shell.extensions.soft-brightness-plus.gschema.xml

@@ -55,9 +55,9 @@
       remains at full brightness.  Controlling mouse cursor brightness
       can sometimes show the wrong cursor and introduce cursor lag.
       You may want to disable it if you encounter cursor issues.  Note
-      that if another Gnome Shell component clones the mouse (like the
-      Zoom accessibility feature), the cursor will follow the screen
-      brightness.
+      that when other Gnome Shell components clone the mouse (like the
+      Zoom accessibility feature), this feature may not work correctly,
+      or there may be multiple cursors shown on the screen.
       </description>
     </key>
     <key name="debug" type="b">