Refactor build tasks to run common task as a dependency

[augustoproiete]

With the introduction of a custom task for cleaning the output on rebuild (CleanExcelAddIn) we've introduced a class BuildTaskCommon that figures out which files are going to be generated in the output.

Whilst this works fine today, it's not very efficient, as the output of this class is needed for both CleanExcelAddIn and CreateExcelAddIn, and gets executed twice when, really, we should only be executing this once.

My current thinking is that we should create a base PrepareExcelAddIn task that will output the files that are going to be generated in the output, and have the CleanExcelAddIn and CreateExcelAddIn tasks depend on PrepareExcelAddIn's output.

As part of the work with #18 and #19 I'm going to introduce a task that updates the debug settings for the project being built, and that task also needs the features in BuildTaskCommon and therefore would benefit from PrepareExcelAddIn as well.