Fixed coloring of headers in TreeViews for dark themes.
Also added a corresponding style for the light themes to make it a little prettier (though it wasn't broken on light themes).
Improve contrast when highlighting parameters in the Onboard Parameters widget for dark themes.
Fix readability issues with the Onboard Parameters treeview items in the light theme.
Merge pull request #804 from Susurrus/master

Fix various styling issues
Run macdeployqt from same dir as qmake

Handles case where multiple versions of QT are installed on machine.
Merge pull request #806 from DonLakeFlyer/InstallerFix

Run macdeployqt from same dir as qmake
Merge remote-tracking branch 'QGS_master_github/master' into dev_merge