Fix GeoPos losing the sign of coordinates between 0 and -1 (#3283)

GeoPos stored a coordinate's sign only in its degrees NumberField, which
is an int32_t and therefore has no negative zero. Any coordinate whose
integer degrees part is 0 but which is negative (i.e. strictly between
0.0 and -1.0) could not be represented at all:

  - set_lat()/set_lon() passed the raw float to the degrees field, so
    -0.2933 truncated to 0 and the sign was gone before it reached the
    widget.
  - lat()/lon() then decided the sign with `field_lon_degrees.value() < 0`,
    which is false for 0, so the value came back positive.

The reporter's airport is at longitude -0.2933, which was impossible to
enter in the ADSB Tx app.

Give each coordinate an explicit hemisphere field (N/S and E/W) and make
the degrees field an unsigned magnitude:

  - The hemisphere OptionsField is the single source of the sign, so
    "negative with zero degrees" is now representable.
  - set_lat()/set_lon() derive the hemisphere from the sign of the input
    and feed the fields the magnitude.
  - lat()/lon() read the hemisphere instead of inferring the sign from
    the degrees value.
  - The minutes on_wrap handlers no longer need to flip the carry
    direction based on the degrees sign, since degrees is now a
    magnitude; the carry is the same in both hemispheres.
  - The degrees fields no longer loop, so carrying below 0 clamps at 0
    instead of wrapping round to 90/180.

The hemisphere indicator occupies the column that the 4-wide signed
degrees field used for its minus sign, so the row layout, the degree
symbol and the decimal readout all stay where they were.

Fixes #3234

firmware/application/ui/ui_geomap.hpp

@@ -239,12 +239,21 @@ class GeoPos : public View {
         {25 * 8, UI_POS_Y(0), 5 * 8, 16},
         ""};
 
-    NumberField field_lat_degrees{
+    // Sign is held by the hemisphere field, not by the degrees field: an
+    // int32_t degrees field has no negative zero, so a coordinate in
+    // (-1, 0) could not otherwise be represented (see issue #3234).
+    OptionsField field_lat_hemisphere{
         {5 * 8, 1 * 16},
-        4,
-        {-90, 90},
         1,
-        ' '};
+        {{"N", 0},
+         {"S", 1}}};
+    NumberField field_lat_degrees{
+        {6 * 8, 1 * 16},
+        3,
+        {0, 90},
+        1,
+        ' ',
+        false};
     NumberField field_lat_minutes{
         {10 * 8, 1 * 16},
         2,
@@ -263,12 +272,18 @@ class GeoPos : public View {
         {17 * 8, 1 * 16, 13 * 8, 1 * 16},
         ""};
 
-    NumberField field_lon_degrees{
+    OptionsField field_lon_hemisphere{
         {5 * 8, 2 * 16},
-        4,
-        {-180, 180},
         1,
-        ' '};
+        {{"E", 0},
+         {"W", 1}}};
+    NumberField field_lon_degrees{
+        {6 * 8, 2 * 16},
+        3,
+        {0, 180},
+        1,
+        ' ',
+        false};
     NumberField field_lon_minutes{
         {10 * 8, 2 * 16},
         2,