Add LineUp column patcher extension

[puehringer]

See #233 for a more information on the problem and why we need to solve it.

Usage
The column patcher is a simple function, included as extension point with the type tdpScoreColumnPatcher. The following example would modify an (already internally patched) NumberColumn score. The actual use case is to patch your own column, which is only accessible from within an app and has an known interface (with methods such as setCategories(...)).

1. Add the extension to your phovea.ts:

  registry.push('tdpScoreColumnPatcher', 'another-number-column-patcher', () => System.import('./patcher/AnotherNumberColumnPatcher'), {
    factory: 'patchColumn'
  });

2. As we have defined the factory: 'patchColumn', we have to create the file ./patcher/AnotherNumberColumnPatcher.ts with an exported function patchColumn.

export function patchColumn(pluginDesc: IPluginDesc, colDesc: IAdditionalColumnDesc, rows: IScoreRow<any>[], col: Column) {
  if (colDesc.type === 'number' && col instanceof NumberColumn) {
    // Calculate something using the rows, and set it on the column...
  }
}

3. Now, each time a score is loaded, your patcher will be called (after the internal patching) to allow your updates to take place.

[thinkh]

I suggest to rename the extension point to follow the new extension point naming schema. The rest looks fine to me.

[thinkh]

Thanks for the changes. It looks good now. 👍