QgsNumericFormat subclasses provide a means to format a numeric value
as a string, applying various formatting options. E.g. a default
string to value formatter (included here) includes settings for
controlling the number of decimal places, whether a thousands separator
should be shown, whether a leading + sign should be shown, whether
trailing zeros should be shown.

This PR also includes a formatter for bearings, allowing various
formats of numeric bearings to be applied (e.g. control over decimal
places, etc + control over whether direction E/W suffixes are shown,
or whether values should be limited to either the +/- 180 range or
0-360 degree range)

When formatting values, a QgsNumericFormatContext class is used
to provide context. Currently, this includes the thousands and
decimal separators to apply when formatting (which are taken by
default from the user's locale).

A registry of formatters is included to allow easy addition of
other formats in future (e.g. currencies, percentages, scientific
notation, etc...) and to allow plugin based formats.

The intention is to follow this up with gui configuration widgets
for the formats, and then expose them in various places through
the qgis ui (e.g. in the range editor widget for fields, in scalebar
numbers, as an option for formatting numeric labels, etc)