Fix some interpolated values of a trackpoint on insertion between points.

Fairly obvious cut and paste error.
Incorrect code introduced (by me) in commit SHA1: db79f75f7deef5e8a934324c69dfaa274764514a
Enable control of create_label_vbox spacing and padding.
Add view of course and speed properties of trackpoints.

Since these values may be held in the trackpoint data, we might as well be able to show them!

Also reduce spacing between the elements to keep the dialog compact.
QA: replace direct access to GtkWidget->window member by function call
[QA] Fix large memory leak when removing DEM layers.
[QA] Fix memory leak in treeview tooltips and selection.
[QA] Fix memory leaks on reading in Viking files.

Don't set these string variables to NULL too early!
Otherwise testing it's value means it previously didn't call the free function.
[QA] Fix memory leaks in usage of the clipboard.

Refactor use of gdk_atom_name() since this returns an allocated string.
[QA] Fix incorrect iterator usage.
[QA] Fix large memory leak on viewing Track Properties.
[QA] Fix new tracks not shown after split called from the Track Properties dialog.
[QA] Fix memory leak on deleting TrackWaypoint layers.
[QA] Fix several memory leaks on program closure.

Not particularly important since the program is ending,
 but it should be done and aids tracing any other remaining memory faults.
[QA] Fix misc small memory leaks.
[QA] Fix name memory leaks in TrackWaypoint operations.
[QA] Fix memory leak in sorting layers.
[QA] Fix memory leak in drawing new tracks/routes.
Fix menu use of named own Icons since the icons were renamed some time ago.

They were renamed back in commit SHA1:79dce0cbecc73aef904c1529467195416c1bfe96
Start to make compression code more reuseable.

Move zip file handling into new a file: compression.c
Unzip comment