Add functionality to the +/- buttons at the top and to the left of the scroll bars

[Anastasia Baryshnikova]

The +/- zoom in/out buttons on the left of the bottom scroll bar and at the top of the right scroll bar are inactive. We should add them the same functionality as the other +/- buttons.

[Robert Leach]

Resolved issue #219. The top and left +/- buttons now work. Also reversed their behaviors based on the interpretation that we are zooming into/out-from the row/col on the side where the buttons are located. Also prevented zoom-out from adding columns on the side that is "fixed" (i.e. the row/col that we are zooming into/out-from). That includes an update to the way the buttons are grayed out.

→ <<cset 5c0554d1be5d>>

[Robert Leach]

The top and left +/- buttons now work. Also reversed their behaviors based on an interpretation from a collaboration with Anastasia that we are zooming into/out-from the row/col on the side where the buttons are located. Also prevented zoom-out from adding columns on the side that is "fixed" (i.e. the row/col that we are zooming into/out-from). That includes an update to the way the buttons are grayed out.

[Robert Leach]

BTW @TreeView3Dev, I added a parameter to incrementalZoom that indicates whether to zoom in on the left or right / top or bottom side. I made it a boolean and called it beginFixed (an abstraction of top/left). It essentially means (if true) that the first row/col is the one we're zooming into/out-from. If false, it assumes the last row/col. The opposing side is where the rows/cols are added/removed.

I also added another parameter called "hardFixed". When true and zooming out, it will only add rows/cols on the side indicated by beginFixed and will not start adding columns on the fixed side after you hit an edge.

[Robert Leach]

Merged to master.

[Robert Leach]

Removing milestone: Navigation (automated comment)