InfoBoxWindow: stop dragging before opening dialog

This is necessary on Windows (TRAC #2634).
Terrain/RasterRenderer: add another local variable
Terrain/RasterRenderer: workaround for division by zero

Workaround for TRAC #2613, because I just can't figure this out.
po: update Polish translation
PageActions: rename namespace to "PageActions"
UIGlobals: move ActivateMap() to UIActions.cpp

These are actions that manipulate the user interface, not simple
access to global variables.
Pan: use UIActions and UIGlobals
tools/lk2po.pl: load ENG_MSG.TXT as UTF-8
tools/lk2po.pl: support Vietnamese
tools/lk2po.pl: delete the obsolete menu code

LK8000 menu translations are now in the regular MSG file.
po: add Vietnamese translation

Imported from LK8000.
LanguageGlue: support Vietnamese
po: update xcsoar.pot and *.po
po: merge missing translations from LK8000
PageActions: add "pure" attributes
PageActions: add functions ShowMap(), ShowOnlyMap()

To replace UIActions::ActivateMap().
PageActions: add functions Restore(), DeferredRestore()

To be used in page widgets that return to the previously configured
page.
PageSettings: add method Compress()

Always ensure that the page array is contiguous.  This simplifies a
lot of page switching code.
PageActions: remove unused function Open()
UIActions: remove obsolete function ActivateMap()