Qt: Dynamic languages

- get language as soon as possible (before creating any Qt objects) to make all
  translations working
- dynamic list of supported languages
- runtime change of GUI language (no need to restart application)
- add flags icons support
- search for *.qm languages in buildin resources, then
  data dir called "languages" (main directory in sources or
  /usr/share/wireshark/languages), then user directory
  (UNIX: ~/.wireshark/languages); "languages" directory should contains
  files wireshark_xx.qm where xx is language code (en, en_GB, etc.),
  and optional xx.svg for flag icon
- try to fix some untranslated manually-created UI items
  (need manual reset text of those components)

Change-Id: I62ca8a8cddce47cec9dbcad6b0bd68b6cfd92229
Reviewed-on: https://code.wireshark.org/review/5041
Tested-by: Michal Labedzki <michal.labedzki@tieto.com>
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>