Slim recent membar colors settings: obey themes [1/2]

Add disabled custom color settings for #00ffffff (fully transparent white).
http://gerrit.aicp-rom.com/#/c/35222/ says "obey themes when disabled", but
there was actually no way to disable it. Now theme colors are taken into account
when user sets color to #00ffffff (just like for the other color settings by slim),
which is now also the default setting.

Change-Id: Iac9dc6a711cbb58df66cca6de6a0ac1049950fae