Parameterize units of MissionItemFields

 - currently, solely for display
 - future starting point for providing option to change between metric
/ imperial etc.
Tagged FIXME with issue num
Minor style adjustments

 - limit to output to two decimal points which is broadly appropriate
(e.g. altitude has cm resolution)
Waypoint/loiter radius no longer uses param3

 - modified alt/time format specs
Move mission item field mutation logic to MissionItemField
Preparing to allow header titles to be modified on selection

 - hold sectionHeader and tag param header labels
 - permit toggling of selected row
Merge branch 'master' into igcs-15-mission-edit
Correct compilation issue

 - prevent definition of _mavlink_send_uart in mavlink_helpers.h
Merge branch 'master' into igcs-15-mission-edit
Moved mission type metadata to utility class
Change param header labels to match selected row
Prevent display of irrelevant params in table
Addressing review comments

 - enough with the gets already! :-)
 - new style boxing and enums
 - more idomatic var and method names
MissionItemFieldType enum -> NS_ENUM
Applying review recommendations to additional code
Further review recommendation adoptions

 - regarding recent obj-c literals / declarative syntax