[Plugins] Print error message when failed to load plugin.

Prints QPluginLoader's error message to the stderr.
Make it possible for plugins to reimplement mailto scheme.
[Plugins] Added possibility to unregister scheme handlers.

It is important to unregister scheme handler when unloading
a plugin, otherwise it will crash on next createRequest call.
[AdBlock] Make it possible to remove EasyList.

Closes #534
[Icon] Fixed incorrect sRGB profile in 16x16 icon.

Closes #843
Merge remote-tracking branch 'upstream/master'
Refactoring private browsing menu action v2