SF Features#124: Enable modifier key to enable panning in select tool mode.

Alt Left on Windows and possibly Alt Right (key in Mod 5 group) on Linux like systems.

Particularly useful on laptops with touch-pads, as using the middle click (to pan) is often quite awkward.
SF Bugs#112: Enable using libgexiv2 for writing geotag image data to void XMP data loss.

Use libgexiv2 which is the C interface to Exiv2 C++ library.
Requires installation of the package named libgexiv2-dev or similar.

If libgexiv2 is not available,
 then it's possible to run configure with --with-libexif to force the use of the libexif
 (and then 'libjpeg' code to write EXIF data only to JPEGs but may lose XMP data)
[WINDOWS] Tidy up Windows build
Shift build configurable bits into modules.c
[QA] Minor function rename for consistency
Support showing a climb (vertical speed) value in the statusbar.

Requires adding 'B' to a format message code.

Presently choose not to store a vertical speed in trackpoints,
 as the climb value is only from GPSD.
On tracks the value is calculated from the altitude and time difference.
Remove not very helpful debug message since it can generate large volumes of messages.
Fix display of calculated speed for the statusbar message.
Improve potential statusbar message of a selected trackpoint.

Actually supply the previous trackpoint so some interpolated values (such as speed) can be calculated.
Fix stdout/stderr variable usage.
SF Bugs#116: Fix erroneous usage of abs(). Use fabs() instead.
Remove configure check for strdup() as it is not used.
Fix potential crashes from long running threads in a removed window.
SF Features#129: Advanced Maps Layer drawing configuration.

Add options to control extent of alternative zoom level tiles
 and whether to try smaller or larger zooms first.

These settings are used in the ~/.viking/viking.ini file.

. 'maps_scale_inc_up' - default = 2
. 'maps_scale_inc_down' - default = 4
. 'maps_scale_smaller_zoom_first' - default = true
Prevent compiler warnings with GLIB version 2.32 or later.

Create new file to contain compatibility functions.
Just GLIB to begin with for GMutex functions.
Extend zoom menus to cater for the highest zoom levels.
Shift GTK+ compatibility definitions into vik_compat.h
Show maps layer tile grid lines in extreme debug mode
Shift Map Ids into a central location for ease of knowing what is in use.
Correct Bing Aerial map name and allow for i18n.