Use the "screen security" preference for the conversation list

It was previously only used for ConversationListActivity. This should be all
the locations (according to a grep for FLAG_SECURE).

FREEBIE
Only receive text/plain.

At the moment only text/plain is handled in RoutingActivity, so set filter in manifest accordingly.
This prevents #316. But at some point sharing text/calendar or better */* would be nice...

// FREEBIE
Merge pull request #1020 from lxgr/fix-screensec

Use the "screen security" preference for the conversation list
Merge pull request #1025 from DorianScholz/sharetextplain

Only receive text/plain.
ConversationFragment: enable back button to dismiss

The ConversationFragment has a AlertDialog for showing the message
details, which sets the cancelable property to be false. This stops the
user from being able to use the back button to dismiss the dialog.
Merge pull request #1053 from shenki/master

ConversationFragment: enable back button to dismiss message detail dialog
Moved colors to the right place.
Closes #913: Message ID is -1 when called in aggregate
Merge pull request #1069 from phenx-de/color-cleanup

Moved colors to the right place.
Give user the opportunity to chose no LED-notification.
Mark message details on pending messages as such.

Closes #1034
Format-neutral compare numbers in MMS group with local device.

Closes #1018