Mouse-only interface for variable operations

[remram44]

(originally TRAC#17, 2013-03-13)

Currently, the only way to perform an operation is to type a command in the right panel of the form:

new_var_name = some_operation(old_var_1, old_var_2 * 5)

Clicking on the operation names will put it in the line edit with highlighted <?> markers for the missing arguments, but then the variable names have to be entered manually.

I think it should be possible to perform a simple operation using only the mouse, by clicking on operations and selecting (or dragging?) variables.

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comment by @remram44:

2bee39e adds drag and drop of variables and replaces the current marker if one is selected. This allows to build an operation only using the mouse.

This might not be perfect but I'm not sure what behavior would be better.

Building a 'VariableDropping'-like overlay showing the operation tree (if the expression is valid) allowing to target a specific parameter with type checking is a possibility, but can be left for later.