skin engine: Fix FS#12884.

The %Vf and %Vb tags change the colors for the rest of the viewport. This
requires the rest of the vp to be redrawn when they change due to a conditional.
The previous code did this redraw in all cases (conditional or not) which
led to visible blinking.

Change-Id: Ie59dfc6fe8ed76485a2a2bd7caf1315f1944c227
playlist_viewer: Resurrect playlist viewer settings submenu in the playlist viewer context menu.

This was lost in 97a4c1ef (svn r30177) for unkown reason but the manual still
mentions this item, so I assume it was an accident. It doesn't hurt anyway.

Fixes FS#12930.

Change-Id: I2f5cd81913ec7bb911d1117e50c010a5c1b89b52
playlist_viewer: Reset icon callback after leaving the playlist viewer settings.

This fixes that the playlist viewer still shows the icons after changing
the playlist viewer specific show_icons value until re-opened.

Change-Id: I42f287bcf02b27f0255dc157560c2e6575307eb6
Print `last logf lines on panic().

If logf is enabled, panic() will print the last lines. On small
screens this is more or less useless, but on large screens it
can be very useful for debugging.

Change-Id: I26dfc76e9ac4a2ddc2def8db1616a04f943dbba3
Reviewed-on: http://gerrit.rockbox.org/709
Reviewed-by: Thomas Martitz <kugel@rockbox.org>
Reviewed-by: Frank Gevaerts <frank@gevaerts.be>
Tested: Frank Gevaerts <frank@gevaerts.be>