activity icon: avoid show background line when use transparent fill
chart: center value in bars
chart: calculate radious and center depending on image size and display of labels
new poll: disable back button in first page
UNscape all the pango markups
new poll: max voters valid between 1 and 1000
Use Pango Layout to print titles, to allow wrap lines

When set a width for the layout, the x, y position is the top left
corner of the layout, _not_ of the text, by example, if the text
is centered.
Select screen: ellipsize title to avoid problems with long titles
poll:Set max width on title and question labels
chart: Set a minimum and maximum size for the title font

Currently looks better on the xo. Not sure why can't have comparable font sizes
on the xo and on my desktop.
create preview scaling the image, to avoid errors with the font sizes
chart: print text using wrap mode WORD_CHAR instead of WORD

This is needed to avoid breaking with really long words without spaces
chart: keep padding between the labels and the pie
poll: set_max_size on radio button labels