fixed bug where override values wouldn't be used if fullDataBounds was
empty

Change-Id: I1c1ce8ed05ac09f78f6579ebc59ddac6b095ca71
GridLinePlotter: renamed 'horizontal' -> 'alongXAxis', and corrected
logic determining whether or not to draw lines based on scale (same in
AxisLabelPlotter).

Change-Id: I5909189a01215c812ff21e0d19decd15ed0ab256
Added buttons for setting/unsetting override coords, and no longer
hiding the controls in map tool (user can now set custom zoom-to-extent
bounds).

Change-Id: Ie67462f315f9f4d4f62ed39e1f8812e5d1cd972b