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.
Update ActivityInfo.qml for all algebra activities

Update the description, author for all algebra activities.
 Merge branch 'algebra' into 'master'

Algebra Plus and Algebra Minus activities

Hi Bruno,

This branch has been rebased with bdoin/gcompris. The thing is, to fork and therefor merge and rebase with developers/gcompris, I'd need to change the path(what git tells me) or delete this fork. I didn't want to do either without asking since I am not a git power user and I wasn't sure. Changing the upstream on my local to developers/gcompris also did not do the trick, as my merge request can only be sent to the parent on the fork here. Will figure things out after asking you on the channel.

The merge request has pushed the Algebra Plus and Algebra Minus activities:
Please note that the commits contain both the implementation ways I tried. I have finalised on one because it keeps the QML separate from the Javascript, but personally, I think the other method is easier to udnerstand. Do let me know what you think.

I have also changed the description slightly in the ActivityInfo.qml file for all the three algebra activities to reflect the use of backspace and the non-use of the Enter keys.

Cheers, and thank you!
normalized imports.

Now all import are QtQuick 2.2, QtQuick.Controls 1.2, QtQuick.Window 2.2

I found out that adding a ComboBox was crashing GCompris. Could
not reproduce on a simple independant program so did not made a
Qt bug report..