presenter v0.5.0

presenter v0.5.0 mainly provides more background styles to show the potential of presenter.

• Add stroke-length key to block and l3block styles.
• New progressbar style to show the page progress based on block style with the new key mentioned above.
• Every foreground/sectioning template implementation now requires to use \pretsec_register_name_title:nn to note the metadata for the use of other modules. Provides more interfaces in pretsec package to enable the communication between the foreground and background templates.
• New secblock style which will decay the color with the increasing of the sectioning level, which demonstrates the communication mentioned above.