Skip non-ICS files when loading from a directory

[thezeroalpha]

Directories containing ICS files may also contain metadata files. For example, vdirsyncer used with the Vdir format creates files such as color and displayname, in the same directory as all other ICS files (see documentation). However, metadata files should be handled separately, and should not be loaded as ICS files.

[anufrievroman]

Thank you!