Add build option to disable new version checking

Closes #1184
[coverity] Followup fixes

webview.cpp: Fixed pointer to local outside scope
bookmarksmodel.cpp: Unchecked return value
[ExternalJsObject] Add window.external Js object to all pages

SpeedDial is now available with external.speeddial
[SpeedDial] Fix issues with loading a missing preview on load

Missing previews are now loaded after the page has been loaded (from
JavaScript code), instead of loading being started on generating
initial script (from C++ code)
[OpenSearch] Added support for external.AddSearchProvider js function
[SearchEngines] Fixed issue with default engine when more engines have same name
[Travis] First attempt to setup Qt4 & Qt5 builds
[Travis] Second attempt to enable Qt4/Qt5 builds
[Travis] Use PPA for Qt 5 + Allow run_scripts.sh to use specified qmake
[Travis] Fixed missing make + caching apt + missing qt51script package
[Travis] Qt5: install gstreamer-dev + build with -Wextra -Werror
[coverity] Fixed Unchecked return value from library
Fixed previous commit

Ooops, it should actually check the return value, not just ignore it...
[ProfileUpdater] Don't overwrite settings.ini on incompatible profile data
[Mac] Disable DBus as it is not available in Qt 5
[Mac] Disable building of QtWebKit-Plugins

It doesn't work as of now and we also don't have linking to hunspell
working either. So let's just disable it for now.
[Mac] macdeploy.sh now also creates a dmg image

Thanks to Seyyed Razi Alavizadeh for create script and images
[AdBlock] Fixed incorrect handling of non-trivial exception rules

News are loading fine now on http://abcnews.go.com/
[Code] Renamed BookmarksModel class to Bookmarks

It actually is not model. BookmarksModel name will be needed for future
Bookmarks DataModel class.
[Code] Various cleanups in Bookmarks code