Allow the user to set arrival threshold

[marikavertzonis]

Now that high precision GNSS receivers are supported (as per #65), the ability for the user to set the arrival threshold is more important.
Consider this a refinement of #3. Some examples are:

• If a user has a receiver that as an accuracy of 1m, and they are searching for a manhole in an open field - an arrival threshold of 1m is a reasonable expectation.
• if a user in a city environement, using either an external receiver or internal position source and looking for a light pole (ie: something obvious), then an arrival threshold of 10-20m is reasonable.

[marikavertzonis]

accuracy threshold should go in the settings section called destination (under longitude) - not a new section of its own - and display the units that are selected below it eg: (m) or (ft) to be clear that even when degrees are used for the coordinates the accuracy will always be entered / shown in m or ft.

[marikavertzonis]

change accuracy threshold to arrival threshold in the above screenshot (my typo)

[marikavertzonis]

with the improvement of arrival mathematics in #3, removing this issue from the 3.0 milestone, but keeping it for future consideration.