Add current mouse position in Volts

Hi, first of all, thanks for making all of this open source, this is much appreciated !

The goal of this PR is to add a text zone on the bottom of the oscilloscope screen to indicate actual mouse cursor's voltage. I needed it to follow some curves with mouse pointer and having precise voltage measurements.

This branch consists of two distinct commits. I tried to minimize changes made to code, not to alter the "spirit" of the code and keep it "consistent". Typically, I resisted to the temptation of making it more close to the MVC paradigm.

First commit

goal: access to oscilloscope gain without having to parse gainText[ui->ch1GainSlider->value()] at each mouse movement

method: I replaced the string array gains definition by an array of structs:

struct Gain {
    float value;
    QString unit;
    QString display;  // human readable version; ex "80mV"
}

advantages:
- efficiency: direct access to gain value, unit and human readable display without need to add CPU load
- clarity: more readable / maintainable code: unit = gains[ui->ch2GainSlider->value()].unit is easy to write and read
  - before:
```
for(int i = 0; i < unit.length(); i++) {
    if(unit[i] == 'm' || unit[i] == 'V') {
        value = unit.left(i).toDouble();
        unit = unit.remove(unit.left(i));
        break;
    }
```
  - after:
```
gain = gains[ui->ch1GainSlider->value()].value;
```

Second commit

goal: add the text zone to display volts value. Conversion from pixels position to volts is done.
changes in XprotolabInterface::moveCursor may looks big, but I only:
- put existing code without any modification in this if instruction: if(event->buttons() & Qt::LeftButton & (currentSelected != isNone))
- added } else if(!event->buttons()) { block to handle mouse move without pressed button

More to-do ?

In the same spirit, we could also indicate cursor's current "time", but I didn't needed it, as I can place vertical markers.

ganzziani / xscopes-qt