Remove old JSON classes.

Conflicts:
	src/webui/json.cpp
	src/webui/webui.pri
Add QJson files.

Conflicts:
	src/webui/webui.pri
Use new JSON parser/generator. Closes #1395.

Qt4: Use QJson (http://qjson.sourceforge.net).
Qt5: Use native Qt JSON API.

Conflicts:
	src/webui/btjson.cpp
	src/webui/prefjson.cpp
Fix libtorrent 0.15.x compilation.
Update search engines versions.txt file. Closes #1745.
Bring mainwindow in front when notification is clicked and other windows are in front.
Suppress warning about signed/unsigned mismatch.
Manually backport from master, commit 6dabf50 and its fixes. Original author Ivan Sorokin.
Manually backport from master, commit 00e09435 and its fixes. Original author Bruno Barbieri.
Fix column resizing issues
Fix possible crashes. Closes #1814.