Readability updates, unifying some magic numbers as constants
Transit button is enabled when ship leaves the gravity bubble and disabled when the ship enters it
Merge branch 'master' into pfs_planetarytransit
A bit of refactoring
Manual transit can be activated and deactivated

When transit drive is active no fuel is used up
Flight control states enum and flight button modes enum are now separate and don't need to match one another
Manual transit now uses transit states to manage operation

Manual transit now behaves correctly when turned off: plays finish sound, restores velocity
Manual transit mode respects altitude better

Several adjustments to get a good feeling for the transit
Manual transit sound
Merge branch 'master' into pfs_planetarytransit
In manual transit drive 2 mode, if ship turns (any angular thruster activated) transit drive drops immediately to mode 1. Restored to mode 2 smoothly once angular thrusters become inactive.
Manual transit switches to maneuver mode when disabled due to altitude dropping below gravity bubble
Fixed transit finish sound playing at random sometimes
Implemented collision tunneling test for manual transit
Drop transit drive down to 1 when mouse control (right click drag) is active, restore when released
Merge branch 'master' into pfs_planetarytransit
Merge branch 'master' into pfs_planetarytransit
TransitAround basic implementation is used for player ship planetary travel (not fully functional yet)

FlyAround is still being used for non-player ships to maintain functionality
Merge branch 'master' into pfs_planetarytransit
Altitude correction works for low altitudes (lower than Transit altitude)
Merge branch 'master' into pfs_planetarytransit

Conflicts:
	src/ShipAICmd.cpp