startcenter: Start center shows a welcome text when recent doc list is empty

The image I got from TDF wiki.

I tried to show the text semitransparent also, but set font color
with any alpha degree (eg. setTransparency(254) ) made it invisible.
Maybe just a gray shade?

Change-Id: Iaf7efb8433e9db03862825148ed2013588a5468f
Signed-off-by: Rodolfo Ribeiro Gomes <rodolforg@gmail.com>
startcenter: Paint either the welcome, or the recent docs.

Otherwise the background will paint over the graphics...

Change-Id: Ic62c563026daa9c4173148cd68652d8aba608089
startcenter: Don't create the Image with every paint.

Change-Id: I90fa5aff94de38d950a78811dad301b91ced89f3