Use libqtxdg for the external editor module

Everything is much cleaner now
GUI fixes
Removed code specific to X11 (using KWindowSystem now) and large code
refactoring to LXQt's style
Use the same actions for both toolbar and tray menu

This fixes the annoying behaviour of the help button's menu
Remove unnecessary separator from toolbar
Fix is not loaded translations
translations: Use install(FILES ...) instead of install(DIRECTORY ...)

SCREENGRAB_QMS holds a list of files not directories.
It succeeds because CMake treats each file as being it's directory. But it
fails when for some reason SCREENGRAB_QMS is reevaluated.
Example:
    cmake .. && make && sudo make install
    cmake -DCMAKE_INSTALL_PREFIX=/usr/local ..

The later call to cmake fails.
Avoid opening two intances of external editor

This happened because of XdgAction connected to triggered() internally
and run the command. A simple disconnect() fixes it.
Drop the check on Qt5LinguistTools_FOUND

Just use the REQUIRED parameter at find_package(). If Qt5LinguistTools
isn't available, CMake stops.
Merge pull request #11 from luis-pereira/use-lxqt-translations-infra

Drop the check on Qt5LinguistTools_FOUND
Use LXQt translation infrastructure

No LXQt dependency is added. It remains a Qt only app.

To update the translations set the CMake command line option
UPDATE_TRANSLATIONS to true. Ex: cmake -DUPDATE_TRASNLATIONS=ON ..
The translations will be extracted and also compiled.
To return to the compilation only mode just set the UPDATE_TRANSLATIONS
option to false. Ex: cmake -DCMAKE_UPDATE_TRANSLATIONS=OFF ..

Warning: When UPDATE_TRANSLATIONS is enabled a clean command will also
clean the generated .ts files. The CLEAN_NO_CUSTOM property should solve
that problem. But it didn't, at least for now. So, after updating the
translations, disable UPDATE_TRANSLATIONS right away.

The .desktop files translatable bits are now split by language in the
translations directory. screengrab.desktop.in holds the non-common, not
translatable parts. The LXQtTranslateDesktop.cmake module glues it all, so
only one file (screengrab.desktop) is installed.

The Qt5TranslationLoader.cmake module takes care of automatically produce
code to load the translations.
Updates main application translations sources
Adds translation infrastructure to the modules

extedit and uploader are now translatable.
Extracts modules translations sources
Replace old command line parser on the QCommandLineParser
Bump minimal required Qt version to 5.2
Remove unused include in moduleuploader class
Remove old unused code
Remove option to use XDG's home config path and use it by default

Done by using Qt's QStandardPaths.
Move configuration const variables to #defines