Updated README.md with SDL2 library
ComboBox: Add ‘down-arrow’
configuration: update to JSON object
Mission Widget: Fix layout issues with mission editing
Mission Widget: Add Default Alt & WP Radius
Mission Widget: Wire up WPNAV_RADIUS setting
Radio Calibration: Protect for array out-of-bounds error
UAS Object: cleanup and addition of MissionOverview object
Actions Widget: Add Preflight Calibration Dialog
Actions Widget: Preflight Calibration Dialog (part II)
Linux Build: Add dependancy on qtdeclarative5-qtquick2-plugin

Shlibs won't find qtquick2, as it is a plugin into qt rather than a shared library.
Graphing: Reworked graphing to include loading TLOG files directly.
Graphing: Code cleanup
Build: Fixes for compiling for (RedHat-style) Linux, removed reference to SDL2main lib

Explicit linking against SDL2main breaks the build on RedHat/Fedora-style Linux distributions, and seems to be redundant on Ubuntu etc.
Joystick: make sure base_mode has been received from UAS before testing it
Graphing: Fixes #529, graph scroll slider disappearing when load is loaded
Map Widget: Fix #530, waypoint numbers not readable
Fixed ambiguous cast that breaks compile.