light theme: make text black
chat window: spread font sizes, add "huge"

The font sizes have been changed to allow for a wider palette of
options:

 * small: 16 --> 14 dp
 * large: 20 --> 22 dp
 * huge:  30 dp

Also, the date and nickname are now scaled to 67% of the chat text.
Revert "implement priority in status dialog"

This reverts commit 436c6bb318702b1b5078ad0399f19280408d3d90.

Client priority is an internal feature of XMPP that is not really
relevant to IM. It is sufficient to keep it well hidden in the
Account Preferences screen, in case a power user needs it.
status dialog: remember old status messages
roster: accelerate unread counters
chat window: run delayed mark-as-read in background
Add icons to the status selection spinner.