Add button and prepare implementation
Implement open folder dialog
Avoid clearing data when axtXpoViewerProject loads
Enable loading multiple files

* Therefore, axtForm_XpoContentSelection is constructed by filesnames array instead of a single filename. The error messages to each single file are collected during load process in loadXpoFile() and displayed in frmXpoContentSelection_Load(), when the form is going to be displayed.
* axtForm_XpoFileViewer handles selection of files or folders and reuses current infrastructure (via axtForm_XpoContentSelection) to get the application elements displayed for selection in the XPO content viewer.
Encapsulate import of single file in new method 'importXpoFile'.