Support choosing both a Source and Target project

[cdfarrow]

FlexTrans uses both source and target projects. Some modules read from the source and write to the target, some just write to the target, and some do neither (see #29).

• Create an ini file option EnableTargetProject.
• Add a Target Project button to the toolbar if EnableTargetProject is True, and store the project name in flextools.ini
• Define a new parameter in the module settings FTM_SourceTarget, which defines the requirements of that module (4 options: NoProject, SourceOnly (default), TargetOnly, SourceTarget.)
• Only open the source &/or target projects that are needed for the run operation (e.g. don't open if a NoProject module is run).
• Pass the source and target project handles to the module's entry point accordingly. (The module's function should take the correct number of parameters: check for this and give a friendly error message if there's a problem.)

Fixes #29