Added documentation and clarified messages to the user.

This feature can be enabled at compile-time and documentation has been added to the README.

Speech synthesis support can now be disabled (DEFINES+=DISABLE_SPEECH)
Speech synthesis dependencies are now checked before enabling.
When support cannot be enabled, the user is now notified.

MAVLink dialects can now be specified as a list, so multiple dialects can be compiled in simultaneously (may result in conflicts between dialects).
MAVLink dialects are checked for existance before support is added.
Users are notified of which dialects are used and which cannot be added and why.
The README now has info about specifying MAVLink dialects.

Now properly checks for if QUpgrade submodule has been cloned.
Allows for disabled QUpgrade compilation.
Altered some code so that compiling without the QUpgrade module could
succeed.
Updated README about QUpgrade submodule.

Conflicts:
	src/comm/QGCFlightGearLink.cc

PX4 config: Remove the specialized throttle handling - this needs testing, but doing the math on paper it doesn’t look like a special case is needed / wanted here.

Use the reversed values for the four control channels as well. Not using them would make the display very confusing.

Enable RC calibration mode during RC calibration to tell vehicle to ignore stick inputs for that period

TeamCity caught this, but I thought the error was from my warnings
work. Turns out it was a real build failure. Windows doesn’t know which
floating point floor to use. In reality the parentheses are in the
wrong place and should be the floor of the divide by 2. But integer
division is always floor, so it’s not needed at all.

Always evaluated to false due to signed/unsigned

Delete of last link would cause crash. Need to check for -1 which
signals no selection in combo box.

Fixing -Wuninitialized

Fixing -Wuninitialized

Not sure what qmake # escape is. So fixed the problem instead.

Also fixed a number of other release build problems