core, commit Balloon.qml and tuxBalloon image

Commit the Balloon timer which accepts a time duration and moves the
balloon from the center of the screen to the bottom of the screen
irrespective of the screen size.
core, add Numpad.qml file

Add a number pad file that creates a number pad of 5 numbers. The number
pad appears as a column
core, modify ActivityInfoTree.cpp and core.pri

1.Add the Numpad.qml and Balloon.qml files to the core.pri makefile.
2.Add the new algebray_by activity to the ActivityInfoTree.cpp file.
Gcompris.pro: Add algebra_by activity.

Add the algebra_by activity to the GCompris.pro file and also sort the
activity fiels by name.
Algebra_by activity: Commit all relevant files

1)Background image is stored in resources folder.
2)Edit the algebra_by/Activity.qml file to create the UI.
3)Update other makefiles.
4)Edit teh activity.js file, though this is still incomplete.