Graph: MOD.MODE mode and vertical drag&zoom

Here's another, rather big commit, which brings the following features:

* Updated to the latest version of QCustomPlot (V1.2.1)
* Added the plotting of the MOD.MODE field. This will show up as strings like "STABILIZE", "LOITER" in the graph at the appropriate time. This is a partial fix for #110, full for #233 and #241, I think.
* Added a way to drag and zoom more than just the horizontal (time) axis:
** Now you can 'select' a graph and drag it horizontally *and* vertically. Using the mouse wheel, will also zoom in both directions.
** Now you can select the axis on the far left (or the numbers, or the label of the data series being plotted) and use the mouse to drag it vertically and the mouse wheel to zoom it vertically. This will not affect the horizontal axes or all the other displayed axes (if you display more than one).
* Added a "big" statistics box (currently available by double-clicking a graph). This box will show the data of the currently *visible* part of the box. This allows you to zoom/drag away the "bad" parts of a log and get data relevant to what is just visible on the screen. For example, this works beautifully to find out the true average of ThrIn, when trying to calibrate hovering throttle. Or to find out the standard deviation of AccZ, when trying to reduce vibrations. Remember - *visible* part only.

Unfortunately, there are some bugs in there still. Every now and then things crash. Especially when one turns on "Show value under cursor". Also, the whole grouping of axes into GroupA/B/C/D might not play well with text-based data like the MODE data series... There's some weird stuff in there that I don't understand just yet.