planegame, major rework of the animations

Replaced arrows in the mobile version by gesture on the helico to
move it.
Now the animation of the plane is smooth thanks to the
SmoothedAnimation.
Gcompris.pro: add algebra_plus, algebra_minus

Also add algebra_plus, algebra_minus to activities.txt
algebra_by: Accommodate algebra_plus/minus

Change AlgebraBy.qml, algebra_by.js to accommodate algebra_plus
and algebra_minus
algebra_minus: New activity.
New activity: algebra_plus
algebra_by: Accommodate algebra_plus/minus

Include Algebra.qml and introduce modularity.
algebra_plus: Create AlgebraPlus.qml
algebra_minus: Create AlgebraMinus.qml
algebra_by: Remove javascript from QML

Remove setOperand() function from the QML and instead use a global
property.
Cascade this change to algebra.js and the AlgebraBy.qml files.
algebra_minus: Use global operand property

Replace JS function in QML qith a global property.
algebra_plus: Use global operand property

Use global "operand" property instead of JS function.
planegame, minor, fixed an undefined reference case.
android manifest, removed splash screen entry.
memory-sound, fixed startup

We where not referencing the right qml. The activity could not be
ran.
menu, minor, added a larger click area

The click area on each menu items now take all the rectangle space.
core, minor, switched an include name
minor, improved serveral icons and backgrounds.
Merge branch 'integration-master' of github.com:hkaelber/GCompris-qt into hkaelber-integration-master
planegame, added a stormy cloud

The stormy cloud give a feedback to the children that is hits the
wrong cloud.
Fixes on ballcatch