Define a specific property for use as separator for handling multiple file selection

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The current 0.4.0 version implementation handles multiple file selection with a \n as a separator between files. This makes it impossible to use an action that requires all selected files as different parameters (in a single command line).

After discussion with @Cirn09, it appears that using a pre-defined property would be the easiest way to provide customization when handling selection with multiple files. For example:

<property name="selection.multi.separator" value="&quot;, &quot;" />
<property name="myarguments" value="&quot;${selection.filename}&quot;" />

For backward compatibility, the property value should be defined by default as \n.

The user manual (documentation) of the new property should also be updated.

The documentation should also specify how to get back to the default behavior/value using the following:

<property name="selection.multi.separator" value="${line.separator}" />

The problem is first discussed in issue #50 and was moved to this issue for clarity, leaving issue #50 for plug-in related discussions.

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The property could be used as a Default Property globally. In this case, it would affect all configurations. For example:

<default>
  <property name="selection.multi.separator" value="&quot;, &quot;" />
</default>

or as a local property specific to a given menu. For example:

<actions>
  <property name="selection.multi.separator" value="&quot;, &quot;" />
  <exec path="myprogram.exe" arguments="&quot;${selection.path}&quot;" />
  <property name="selection.multi.separator" value="${line.separator}" />
</actions>

In the previous case, the property expansion of ${selection.path} might be wrong (use previous selection.multi.separator) since "selection" properties are updated right after the user has clicked on files and before processing actions. We may need a way in the system to detect a change in properties that requires all other properties to be re-evaluated.

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A possible solution would be to change how the system defines the property selection.path in the first place. I think it should always define a multiple selection and use ${selection.multi.separator} instead of using \n. When an action would require to have access to ${selection.path} it would also properly expands the separator. This is worth the try.

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The issue is now integrated into master branch. Thank you @Cirn09 for proposing this feature.