[DOC] Format acquire's reference documentation
[DOC] Reformat viktrack's reference documentation
[DOC] Improve babel's reference documentation
Simplify acquire feature

Remove type member from VikDataSourceInterface and use only process function.

Signed-off-by: Guilhem Bonnefille <guilhem.bonnefille@gmail.com>
[DOC] Improve aquire's reference documentation
Allow to download and acquire a GPX without gpsbabel

Like a_babel_convert_from_shellcommand, when input_file_type is NULL
gpsbabel is not used. Instead, the file is read just after download.
Avoid gpsbabel conversion on OSM traces

Disable the gpsbabel conversion as retrieved data is already GPX.
[QA] Remove unneeded include statement.
[QA] Remove unused viewport variables.
[QA] Tidy up preferences.c
Add tooltips for Preferences and Layer Parameters.

Only set a few tips, as most values are self explanatory.
Align all VikLayerParam data usage and use better index referencing.
Change TrackWaypoint properties that are radiogroups into combo boxes for compactness of displayed dialog.
[QA] Add specific enumeration for route finder tool
[QA] Better variable name usage following the change from name to an id.
Optimise: get reference to VikWindow only once per TrackWaypoint layer drawing update.

Also makes the code a little more compact.
[QA] Use specific type GHashTable *, instead of generic gpointer.

Not quite sure why I thought I couldn't use the proper type at the initial time of writing this code!
Probably got carried away as tracks + waypoint types can't be used directly.