micro cleaning
switch to "natz" for handling timezone

merge pytz and tzlocal codes into my own library "natz" (native timezone)
with modifications and simplifications
no more dependency to pytz package
native on unix (use /usr/share/zoneinfo)
not working on windows (yet)
bug fix in TimeZoneComboBoxEntry: menu items were empty
remove pytz from debian dependencies
bug fix in tags.EventTagsAndIconSelect
add dependency to old gobject module in install scripts
merge branch next into py3