Sensors: add parameters to calibrate the z-accel

This is redundant with the 3 parameters of the accel bias,
but by applying it as a separate term it avoids repeating
the six-point calibration on a regular basis.

This is useful especially for VTOL aircrafts where having a
good z calibration is especially critical for a good autonomous
hover.
Calibration: calculate the z-accel offset when leveling

This calculates the offset that will make the z-accel average
gravity after leveling.
Calibration: connect the bias leveling method to a button

The user can either click a button to calculate the board rotation with
or without zeroing the z-accelerometer.
Attitude: move the zero gyros option check box

It was grouped with the leveling button (historically) but is
an independent option so doesn't really belong there.
SetupWizard: use bias and leveling for MR

This method is better for multirotors as it results
in a better hover.
Eagle Tree Pilot Tube Sensor: Fix calibration and airspeed computation.

The sensor was being miscalibrated because the division was wrong. This
fixes the issue, as well as cleans up the defines.

It also changes the airspeed calculation to occur only when the pressure
is positive.
Add style sheet for inverted horizontal sliders.
Refactored input and output channel reversing. This involved
simplifying the code, as well as removing checkboxes and
replacing with less ambiguous visual clues.
Remove unused reverse button.
Changed behavior so that the spin boxes would wait for the user to finish
before updating the neutral slider.
Added swap icon.
Added reverse icon button.
Output config: Automatically set the maximum spinbox values for output PWM pusle width.
BoardManager: Add virtual keyword to getChannelBanks()
OutputChannelForm: Change to qint32.
Output config: Resolved race condition

Previously, a race condition would occur when the ActuatorSettings UAVO
was updated before the board was recognized. This led to the spinbox
settings not being updated correctly.
Changes requested in review.
Merge pull request #1001 from peabody124/level_calibration

Level calibration
Merge pull request #760 from kubark42/reverse-channels-button

Reverse channels button
Merge pull request #980 from kubark42/output-pwm-maximum-as-function-of-channel-frequency

Output pwm maximum as function of channel frequency